A pkm has 3 abilities, and it stores which abilitynumber (0/1/2) as well as the actual ability ID. The Batch Editor can't [generically] set the actual ability ID that corresponds to the ability number, but will have the capability in the future.
Different species have different abilities, so:
.AbilityNumber=4
.Ability=128
Will set all PKM's abilities to Defiant and says to the game that they have their hidden ability. The game only cares about which abilitynumber when it evolves (to pick the new ability ID).