WoW:API UseAction

UseAction (ID,#{,target})

The first number is the action ID.

Each button on your action bar has an ID. The first button id is 1, and so on. There are 6 action bars in all, thus the highest action bar is 72. With the addition of ver 1300, Page 3 acts as the Right Action bar, page 4 acts as the second Right action bar, page 5 acts as the Bottom Right bar, and page 6 acts as the Bottom Left bar.

The second number is either 1 or 0. The third number is optional and represents whether the action should be executed on yourself (1 = on yourself, 0 or nil, on your target or give you the spell targeting cursor). The default appears to be 0.