There is debate in the Type community about whether the tertiary function is in the same attitude as the dominant function. Me -- I come from the school that believes it is. Which means for INFJs the tertiary would be Ti (introverted Thinking), and for INFPs the tertiary would be Si (introverted Sensing).
Let me share some descriptions of what these functions look like:
Here are some more concrete examples of the two. See how comfortably these fit you:
I compare current experience against stored past impressions and experiences.
I am aware at a detailed level of what is going on in my body, including my emotional state.
I seek precision in the use of words and in my internal models, frameworks, and blueprints.
List everything you need to take on a trip and organize them into categories based on where they will be packed.
Now here are some ways I've seen these play out in real life.
INFPs are often well rooted in the past, with a good memory of things that have happened historically, and are attracted to "collecting" facts. They sometimes have a strong grasp of family and tradition and may have a great deal of familiarity with their family tree or display an interest in genealogy. They may act as the "family historian" at times, knowing who begat whom and how a particular branch is related. They have long memories. I know one INFP who could recite from memory what year of childhood he first spoke or began walking, including details about what hairstyles he wore at different stages of life! Others like to visit the same places frequently, and let their minds drift back in time to recall previous visits. Nostalgia is often engaged, and they enjoy telling stories from their past. (Introverted Sensing is a cultural norm in the U.S., so it seems fairly "typical" to look to the past this way.) One INFP tells how he enjoys his routines, his habits, doing things by rote. He does the everyday chores of maintaining his home, and these keep him in his comfort zone. Similarly, sometimes INFPs will stay at the same job for years because they're used to it -- even when the job stinks. (A friend of mine stayed at a dreadful job for 9 years!) INFPs sometimes display "body wisdom," and their bodies can be barometers for how they are feeling, whether through illness, food allergies, or headaches. They are often given to taking medications to control their physical being in some fashion. (On a Yahoo Group one time when there was a discussion of medications, it was a veritable drugstore! It seemed every one of them was popping pills to control something.) These are all common ways that Si will manifest in the INFP type code.
I suspect that many INFPs believe they are "J's" because they enjoy the predictability of a routine, and imagine that is "J."
(If you're an INFP reading this, perhaps you can share with me some ways in which you manifest your introverted Sensing...)
INFJs, on the other hand, are drawn to categorizing. They display a knack for systematizing and can do it quickly. They typically define terms, or ask others to define theirs. They expend a great deal of effort finding precisely the right word to express what they want to say, sometimes rewriting emails many times over until they are succinct, hopefully pithy, and all the words seem exactly right (like not saying "invoke" when you mean "evoke" or "conscience" when you really mean "conscious" or "wretch" when you mean "retch"). They often behave like English teachers, and frequently get asked to proofread others' writing for errors. They can be highly sensitive to misspellings or it's/its mistakes on public signage. They strive for clarity and precision, and often contribute these gifts to a conversation. They attempt to articulate things unspoken, and name aloud any problem or peculiar dynamic. They may enjoy methods of organizing, such as the systematic "Color Me Beautiful" approach to fashion-dressing, or database designing, and of course, Personality Types. Some of them are professional organizers or financial planners thanks to their tertiary. It is the INFJ who will relentlessly re-take a personality test to grasp the categories better in order to figure out which category they really belong to, and become irritated if the results change each time. The INFJ cares more than the INFP does about people being "mis-filed" with the wrong type code, and I know an INFJ who got disgusted when her Step II inaccurately reported her as an INTJ. They are unwilling to settle for INFx (I confess: that designation makes me crazy). These are common ways that Ti will manifest in the INFJ type code.
(If you're an INFJ reading this, perhaps you can share with me some ways in which you manifest your introverted Thinking...)
Now in contrast, INFPs may get curmudgeonly if they are pushed to condense their writing, or if too much is made of particular categories. I've even seen them get hostile over having their spelling corrected! Once I was castigated by an INFP who got angry when I said I felt a certain type list should be exclusive to posts for that type, and she took it quite personally (not being that type). Another time I challenged an INFP's definition of a cognitive process, and she jumped down my throat and unleashed a tirade on me rather than discuss the question calmly. It seemed disproportionately hostile, and abruptly ended a dialogue I had expected to relish having. (Mind you, this is the only person I know who can facilitate a discussion about religion in polite company, so her diplomatic skills are normally astonishing.) Because Ti is in the 8th position for INFPs, they can get downright "devilish" when a definition is challenged or when categories feel oppressive or restrictive to them. They prefer to define things their own way, and a little fuzziness doesn't seem to bother them. (Of course, discomfort with categories is probably one reason INFPs often mis-type themselves. To give primacy over to detached, impersonal categories would entirely undermine using subjective values, their favorite function.)
I've also seen INFPs revel in the past and spend much time on "Memory Lane." Recently I caught up with an INFP I hadn't corresponded with in several years, and all the chat was about the "old days" and wallowing in memories of the past. When she crafted a post to an email list, it invariably referenced her personal history -- either her educational background, failed relationships, past experiences -- these were always her come-froms. One time she posted regarding a bout of depression she suffered a decade ago, and the very memory made her cry. (Introverted Sensing isn't simply about remembering something; it's about recalling an experience in every way -- actually re-living it as if it is happening again for the first time.) This same gal also devoted quite a bit of energy comparing her current email list to an email list she used to belong to (Si does comparing). One time she produced a set of "rules" when she thought a list-member was going out of bounds (even whilst claiming she could never be as rule-bound as an SJ). She was astonishingly unaware of how much time she spent living in the past. All her frames of reference seemed to be about yesterday, and what had changed. When given a choice between something old and something new, she seemed to reject the new in favor of the known.
In comparison, INFJs tend to possess the memory of a sieve when it comes to their personal history. (I repeat, this is not about memory per se; it's about a frame of reference to the past. Everybody claims to have a bad memory.) Now, I can barely recollect what I did yesterday, much less something I did as a child. When somebody asks, "How have you been?" or "What have you been up to lately?" my mind goes blank, even if I just got back from a trip (I did!). My childhood memories are so vague that I once wondered whether I had been badly harmed as a child, since that is often a symptom of abuse. My ISTJ sister recalls lots more than me about my own childhood, or related facts. (Trust me, she would know all the details about any abuse.) Similarly, I lack interest in genealogy. I attended a family reunion one summer, and I was really impatient with figuring out how I was related to anybody there. Whenever my mother starts reciting "begats," I nearly pass out. When she tries to "catch me up" on news about a high school classmate, I usually don't know who she's talking about and could care less. I'm far more comfortable telling you how I'm *going* to be tomorrow, next week, or next month than I am telling you how I was yesterday (really!). When I re-connect with somebody and they want to reminisce at length about the "good old days," my heart sinks and I want to escape. I became a coach rather than a therapist because I couldn't stand the idea of hearing about people's childhoods and past histories all the time. (Virginia Satir called such conversations a "trip through the museum.") Because Si is in the 8th position for INFJs, they can get downright "devilish" when they're asked to reference past events for extended periods of time. And don't ask me how oppressed I feel when somebody tries to control me with rules! Airport security alone nearly makes me hyperventilate. Worse, anything that looks like a routine or a habit has me running the other direction -- I don't even like taking a daily vitamin, and heaven help me if a doctor puts me on a medication of any kind. I've never been able to brush my teeth regularly -- or do anything else "regularly," for that matter. And I mean nothing! Both my husband and I keep an extremely erratic schedule, and we also steer clear of day-to-day chores. Whenever we try to pack for a trip, it's a nightmare because we can never find where something got put away from the last trip (it probably didn't!). For that matter, I can't even stand going to the same job everyday -- I never worked at one job long enough to qualify for a paid vacation. However, I did love the novelty of being a temp and going to new offices all the time and meeting new people. Two weeks at the same job is just about right for me, and then I start craving something new.
If you can't decide whether you're an INFJ or an INFP, you might take a closer look at how your tertiary function manifests, and see how well it fits one of these patterns. This is becoming one of the most reliable measuring sticks for noticing INFP/INFJ differences (especially online), so I urge you to spend some time with it.