ProDOS was an
apple II series
operating system that replaced
DOS 3.3. It had a number of advantages over the earlier operating system.
It was later renamed to ProDOS 8 and a new version was created, ProDOS 16 for use on the Apple IIgs.
The '8' in ProDOS 8 refered to the fact that the apple II had an 8 bit processor(either a 6502 or a 65c02).
The '16' in ProDOS 16 refered to the fact that the IIgs had a 16 bit processor(a 65816).