Here is the listing of NEC linear reg device types.
Generally, for single output NEC 3 terminal thru-hole fixed
character A following type number = output max current up to 2A.
character L following type number = output max current up 100mA
character N following type number = output max current up to 300mA
character M following type number = output max current up to 500mA
no intermediate character means output max current of 1A
The characters following the xx part are not explained anywhere but it
can be deduced from the datasheet that the 'A' (as in uPC78MxxA)
refers to a better line/load regulation than a standard reg. (as in
uPC78Mxx which are no longer manufactured I think).
The appended 'H' (when following the 'A') does not appear to mean very
much other than a package type either TO126 or MP-45G. There is
sometimes an 'F' after the 'H' but there does not seem to be any real
significance for the end user in this character.
All in all, the appended characters don't mean anything you should
concern yourself with. The real characters which mean something to the
user are the intermediate ones.
Hope this explains everything. If not, then you should get further
explanation from NEC.