WoW:ActionSlot

Action Slot IDs can vary for some classes and are completely arbitrary to the buttons they exist in.

Default UI use of Action slot IDs

ActionBar page 1: slots 1 to 12 -- Note exceptions below for other classes
ActionBar page 2: slots 13 to 24

ActionBar page 3 (Right ActionBar): slots 25 to 36
ActionBar page 4 (Right ActionBar 2): slots 37 to 48

ActionBar page 5 (Bottom Right ActionBar): slots 49 to 60
ActionBar page 6 (Bottom Left ActionBar): slots 61 to 72

ActionBar page 1 Battle Stance: slots 73 to 84
ActionBar page 1 Defensive Stance: slots 85 to 96
ActionBar page 1 Berserker Stance: slots 97 to 108