When an opponent goes to conquer an open battlefield and gets priority, can they use their action to play Bullet Time on a different battlefield, or must they resolve entering the battlefield first and only use actions for that battlefield?

Ruling: They can use Bullet Time (or other spells and abilities) to target any valid place, including a different battlefield. Targeting restrictions to the specific battlefield only apply when hidden cards are played.

Nuances:

• The restriction to target only the current battlefield applies specifically to hidden cards being played
• Spells and abilities with priority can target any valid location