What are the differences between GE arc-fault breaker models?
It's a generational design difference

The early generation AFCIs (all branch/feeder units, and the earlier combination-type units) relied on a ~30mA differential (equipment protection ground fault) trip to detect arcs to ground.  While some still do this to the best of my knowledge, this does mean that you need two-pole AFCIs for shared neutral/multi-wire branch circuits, and that's not something that people "get", so to speak, leading to quite a bit of befuddlement and false tripping.

So, the GE engineers went back to the drawing board, and figured out how to make an AFCI that can detect arcing to ground using the arc detection logic, eliminating the need for a ground fault trip.  This allows a pair of handle-tied single-pole breakers to be used on a two-pole circuit, and is the design change denoted by the "AF2" suffix (vs "AF") in the catalog number + the "MOD 3" designation, as per this GE appnote.