Obviously, N is from the Neue Klasse. The letters A, B, C, ... group models by classes, but it doesn't necessarily mean that class B is larger than class A, so this is probably based on some of their internal nomenclature. So, all models under A are from the 3 Series class, B models are from the 1 and 2 Series class, and so on.
NAx:
NA0 - i3
NA1 - i3 touring
NA2 - i4 coupe
NA3 - i4 cabrio
NA5 - iX3
NA7 - iX4
NBx:
NB0 -i1
NB8 -i2
NB5 -iX1
Also under B, there will be iX2
NCx:
i5, i5 touring, iX5, iX6, and maybe some other models....
NDx:
ND0 - i7
and likely i8, iX7, iX8(iXM)
These are my guesses, based on data from @ynguldyn posts on bimmerpost.com forum.