Can something like this exist? I've seen a couple places where people list their type as something like INTJ with INFP subtype.
What does it exactly mean to say someone has INFP subtype? Would this just be an INTJ who is closer to the middle on F and P? Or is this someone who has INTJ as a primary type but takes on much of the behaviors and thought processes of INFP?
Other personality typing systems have explored subtypes. In enneagram you have wings and also variant stackings. There is also the tritype model, so you can think of yourself as having secondary and tertiary types in addition to your primary one.
People speak of subtypes in socionics as well. You have a base type, but with one of the functions being enhanced, you get a type such as ENTp-Ni. This would be an ENTp with enhanced emphasis on usage of Ni.
I think in the MBTI system, subtypes would add more nuance and help explain why someone is one type but often identifies quite a bit with a different type. It would explain why some INTPs appear rather ENFPish while some are more ISFJish, etc. I suppose with this theory you could even an INTP with ESFP subtype. I imagine that would be more uncommon since they share no common cognitive functions and only have P in common.