Monsters are invincible/Algorithm question???

The X register usually holds the attacker ID, a number from $00 to $14, for each of the ten possible entities in battle. For PCs it is $00, $02, $04, or $06, so CPX #$08 and BCC is often used to differentiate between characters and monsters. For targets, the Y register is used instead.

I actually have a question about this. I suspect there are quite a bit of "ID" numbers that I'm unaware of. Where can I look to find out this sort of stuff? Are those variables stored in the banks like all the other information or are there other documents I should be looking at?