People are skeptical of more than two subtypes but I think determinating DCNH is actually more useful than choosing between the two main subtypes.
These IEI-Ni and IEI-Fe subtypes are good for people who clearly fit into them. So Dominant IEI (IEI-Fe) and Harmonizing IEI (IEI-Ni) will have no problem choosing the subtype. But as I see it, in DCNH system you are probably either Creative (IEI-Ne) or Normalizing (IEI-Fi) subtype and that's why you can't decide.
At least it worked for me. I felt like I was often behaving like extraverted person but still I was too lost in thought, too irrational to be Fe-IEI. I seemed not to rely so much on Fe but more on Ti. And on the other hand, in public I was able to behave in a really open, even goofy manner, too extraverted to be Ni-IEI. So I ended up being Ne-IEI which at first seemed weird for me. What my ignoring function has to do with me? But then I started to realize that it really made sense.
So if you want to specify your subtype, I suggest you should check out the DCNH system.
I don't know if it helps but:
If you're extraverted introvert (contact), you could be Dominant or Creative subtype.
And if you're introverted introvert (distant), you could be Normalizing or Harmonizing subtype.
If you feel more rational (terminal), chances are you're Dominant or Normalizing.
If you're irrational (initial), you are either Creative or Harmonizing.
You'd better check the original descriptions and think about it on your own.
It may seem complicated but it makes sense after all.