If I hold a location at the start of my turn and later move the unit off that location, can I play a unit from hand to that now-open location?

Ruling: You need to control a location for it to be a valid location to play a unit to. If you don't have a unit at a battlefield (unless that battlefield is currently in combat and you started the combat with control of the battlefield), you don't have control of that battlefield. Therefore, once you move your unit off the location, you can no longer play a unit there.

Nuances:

• Exception: You maintain control during combat if you started the combat with control of the battlefield, even if your unit is no longer there