About the Order
Xx — Dominant
Xx — Supporting
Xx — Third
Xx — Fourth / Inferior
The order of a stack is decided by which functions the person prefers to use. From first to last, these are the functions that come most naturally to the person. Much like being left or right handed: a person might be able to use both hands, but usually has a preference for one. Some are better at using their "bad" hand than others. Some have practiced more. Some have lived their wholes lives using their "bad" hand and just got set free by learning their type.
Each function, in a way, submits to and serves the functions above. An ENTJ (Te, Ni, Se, Fi) does have Se, but their Se serves more than leads.
Don't get too Nit-Picky
It's important not to focus TOO MUCH on the order of the stack. It's hard to really compare extraverted and introverted functions against each other, for one thing. We've chosen to not go into further detail than a general order for this reason, like assigning percentages to each, or making a test to measure the "strength" of each function. That would go against the original intent of Type in Mind, as we're developing a language to help people communicate, not a system to measure people's souls.
I have an INFJ friend who is really into her Ti. [INFJ = Ni, Fe, Ti, Se.] She had a lonely childhood, and was very introverted for those years. She hypothesizes that she just spent a lot of time developing her introverted functions (Ni and Ti) and is very aware of them. Extroverted functions, particularly in introverts, can be harder for the person to notice; all the people in this INFJ's life would say, "Oh yes, you're DEFINITELY Fe!" but she's often more aware of her Ti than her Fe.
We could get super nit-picky and try to figure out which is really higher out of her middle two functions, but would it really be worth it? Is there that much benefit to knowing that? Or I should probably say, is there that much benefit in expressing that through the person's stack? Probably not.
First (Dominant) Function
This is the person's "main world". As an ISFP (Fi, Se, Ni, Te) I spend a lot of time using both of my top functions, but I'd say that my main world is Fi. Thinking about stuff, deciding how I feel about it, seeing how things line up to my values, processing emotions; this is my life.
I like to think of the first function as the grand-daddy of the stack. My Fi is my wisest, strongest part.
Second (Supporting) Function
Often called the Supporting Function, the second function on a person's stack is really important and strong. The first two functions team up and form most of the person's personality.
If you want to create a super simple character, give them only the two dominant functions. Many cartoon personas have two distinct functions, but you can't see much evidence of the third or fourth. The supporting functions come in and make the character more "human". What is a Te-dom bad guy with no sensitive Fi? Or a an ISFJ heroine with no Ti? Flat, that's what they are.
A person's third function often works hand in hand with their second. Because stacks are always JPPJ or PJJP, the middle functions are either both judging or both perceiving. They're like a married couple, or those two awesome kid brothers that you see working together in like, every movie.
The third function also serves as the brakes. Let's say an INTP (Ti, Ne, Si, Fe) is invited to spontaneously go to the beach with their friends. Their Ne might get excited, but their Si is quick to quietly remind them, "Remember last time we went to the beach? The sand made you itchy for 17 hours, and there was no wifi. Are you sure you want to go?" Rebecca says that her Si doesn't just remind her of the last time she went to the beach, it reminds her of every time she's ever been to the beach.
The third function can also provide relief, or a "way out". For an ESTJ (Te, Si, Ne, Fi) their Si might be quite comfortable doing the same thing again and again, but their Ne says, "Hey, what about this new idea! We could do it differently!" While their Si is faithfully trudging on with daily life, their Ne helps give them vision for the future and see new possibilities. For me, my Ni (third function) figuratively pats me on the shoulder and says, "It's okay if we don't understand everything concretely. Sometimes things just don't make sense."
The third function doesn't control or lead, but offers its strength and support to the functions above it.
Fourth (Inferior) Function
As the least preferred, and probably least developed function in the stack, this is often a person's achilles heal. However, it can be a hidden strength.
If the first function is the grand-daddy, the fourth is the little kid. It's like a really intelligent third-grader: super smart, top-of-their class, but still a third-grader.
As an Fi-dom, my inferior function is Te. It's like a weapon or tool I pull out when I need it. Sometimes it rises as my defender. It's not my strongest thing, but it's still pretty good. I actually feel pretty confident using my Te, since my dad (ESTJ) celebrated and encouraged any Te stuff he saw in me. I grew up practicing Te, basically, so it's pretty well developed.
Still, Te is my achilles heal. It comes out in a negative way when I'm stressed, tired, or hungry. When I'm feeling burnt out, I find myself on Facebook critiquing everyone's posts:
"How could they think this meme is funny? The joke wasn't even delivered properly. Great idea, but terrible finish. Ruined. It would have been great if they just changed this one thing."
Te is also something that intimidates me in other people. It can even be a bit scary. "They're so intense! They're just like, doing stuff!" It can be harder to get along with people who have the same functions as you in a different order, than if they just had totally different functions. It's like they're doing the right things, but their priorities look out of whack.
A couple other examples: To a Te-dom guy, their Fi might be like a little girl that tugs gently on their sleeve and whispers, "Don't forget, people have feelings." To an Ne-dom girl, Si helps her feel grounded.
So, in Summary
1st — Dominant Function (Grand-daddy)
2nd — Supporting Function (Teams up with 1st and 3rd a lot)
3rd — Third Function (Serves the top two, acts as brakes or relief)
4th — Inferior Function (little kid)