Can you flip a hidden card at one battlefield in response to an effect targeting a different battlefield, even if the hidden card's effect can't target that other battlefield?

Ruling: You can flip a hidden card in response to effects at other battlefields, but the hidden card can only affect things at the battlefield where it is located.

Sequence:

• Hidden cards can be flipped at reaction speed in response to effects at other battlefields
• When flipped, the card must target something at its own battlefield (if it requires a target)
• Units do not need a valid target to be played, so they can be flipped even if their effect won't be useful

Nuances:

• Hidden cards include reaction speed as part of the hidden keyword
• A hidden card at one battlefield cannot affect things at a different battlefield
• For units specifically, they can be flipped without needing a valid target, but any targeting effect must still target something at the same battlefield