When a card effect plays a unit from exile, at what step is the unit's location (base/controlled battlefield) selected, and is this considered a 'choice'? Does placing the unit on the chain give the opponent priority to respond with a reaction?

Ruling: When an effect plays a unit to pending, you choose which unit to play and put it in pending, but do not make choices (including location, costs, targets) until after the current ability resolves. When the unit becomes the first item in the pending queue, you finalize it into the chain by making all choices and paying costs. Once it hits the chain as a permanent, no one gets priority per the rules of permanents on the chain, and it resolves.

Sequence:

• Choose which unit to play and put it in pending (no other choices made yet)
• Current ability finishes resolving
• When unit is first in pending queue, finalize it: make all choices (location, costs, targets, etc.) and pay costs
• Unit hits the chain as a permanent
• No priority given (permanents on chain don't allow reactions)
• Unit resolves
• If other pending items exist, repeat finalization process

Nuances:

• Selecting a unit's location is considered a "choice" that is made during the finalization step, not when initially putting the unit into pending
• Despite being a new item on the chain (which normally allows reactions), permanents follow special rules that prevent priority from being given