Can hidden units be played to any controlled location, or only to the battlefield they are hidden in?

Ruling: Hidden units can only be played to the battlefield where they are currently hidden. Their effects can also only target units at that same battlefield.

Nuances:

• This restriction applies to both the playing of hidden units and their abilities (e.g., Blastcone Fey's play ability must target units at the same battlefield where it is hidden)
• Hidden spells follow the same rule - if Hidden Blade is hidden at a battlefield, it can only target units at that battlefield
• However, if you have a card like Hidden Blade in hand (not hidden), you can cast it as an action to target any unit at any battlefield
• Hidden cards are subject to the context of the battlefield they are in for all choices and targets