Rule 170.11

Battlefields can be referenced in different states in card text:

• 170.11.a Battlefields can be “occupied.” This means they have a Unit present.
• 170.11.b Battlefields can be “uncontrolled.” This means no player controls them.
• 170.11.c Battlefields can be “open.” This means they are unoccupied and uncontrolled.

Official Core Rules