For Rumble Hotheaded's ability, can you recycle a unit from anywhere, or only from the board?

Ruling: You can only recycle a unit from the board (base or battlefield). "Unit" without further specification only refers to units on the board.

Nuances:

• "Friendly unit" means a unit you control, which excludes opponent's units
• Units in hand and trash are in non-board zones and are not considered "units" for this purpose
• The term "unit" by default only refers to objects on the board unless specified otherwise