Do hidden cards at battlefields only interact with units at that specific battlefield, or can they target units anywhere?

Ruling: Once cards are hidden at a battlefield, they can only target things at that specific battlefield.

Nuances:

• Zhonya's is an exception where rules-as-written currently allows it to protect all friendly units globally even when played from hidden, but rules-as-intended is that it should only affect units at the battlefield it was played from
• The Zhonya's interaction is acknowledged as not matching intended design and may be clarified in future rules updates