I want to skip all the subjective added stuff and just cover the basics.
Functions can not exist by themselves. There is no such thing as without either or . So we can just call it " blocked with " and " blocked with " (in the same block with, not blocked/stopped with). Or we can use a different nomenclature and say - and +. It makes no difference. But according to the model and rules of socionics we can be certain that Ni is always blocked with either Fe or Te and that can not be blocked with , , or .
There is a difference between +Ni and -Ni. Ni is always blocked with some mixture of Te and Fe and everyone has both functions. But some functions are easier to use than others. And it seems some function combinations are easier to use than others. For ENFjs, Fe is much easier to use with Ni than with Si. And when I use Te, it's easier to use it with Si, right? (resulting in -Te and +Si). ego block functions are the strongest and those are most clearly either + or -. It's not easy to jump from using your creative to using your PoLR, which means your program function is clearly blocked with your creative. There is no point in saying that a person can easily start using +Ni in stead of -Ni, because they became more confident in something. It is more likely that their -Ni will look like +Ni if not observed closely. And if it's systematic behavior, it's more likely that it has always been +Ni. I say that because a person can not start using their PoLR every day without any problem. ILI with +Ni would have to use Ni blocked with Fe, which is absolutely absurd.
The controversial theories of the loud and unreasonable hitta has currently made the entire +/- theory controversial. I do still think that understanding the manifestations of functions through other functions can give further information about types and relations. I think that +/- functions explain a new aspect of some relations being better than others. For example it explains why semi-duality and illusion are not very good relations even though one function is correct. And if I'm not mistaken, it also explains why conflicting and super-ego relations have such a strong initial chemistry - they have the right kind of functions.