- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks whether you are acting through the action bar you choose: your own, a vehicle, a possessed unit, an override bar, or a pet battle.
Why you might use it
Use this in a switch condition to give a different action bar a rotation of its own: a vehicle, a mind-controlled (possessed) unit, an override bar that a quest or encounter puts up, or a Retail pet battle. A rotation normally runs only on your own action bar; once its switch condition requires another context with this condition, it is used, and its buttons are found, while that bar is up. Vehicles and override bars exist from Wrath onward, and pet battles only on Retail.
Example
you are acting through a vehicle
Settings
What each setting in the condition editor means, from the game's own help.
Matches the bar you are acting through: your own, a vehicle, a possessed (mind controlled) unit, an override bar, or a pet battle. Vehicles and override bars exist from Wrath, pet battles only on Retail.
A rotation of your own is used only on your own bar unless its switch condition requires another action context with this condition; it is then used, and its buttons found, while that bar is up.