The A/ACC and B registers are located in the SFR area, thus making access possible when no dedicated instruction exist. A refer to "use as a register" and ACC refer to "use as a SFR"

E.g. Push a does not exist, but push ACC does. Conversely there is no add ACC, but an add A. If you in chapter 2 of "the bible" see something like
MOV A,#data
the use of 'A' is evident, but for
MOV direct,#data
you can substitute ACC for 'direct'

now, there is no
push A
but a
PUSH direct
which you can use as push ACC.

I have not looked into this for ages, but without any investigation I'll say use 'A' when possible, use 'ACC' when not.

Erik