A third region can be a witness

A witness is a voting member that keeps no full copy of the data. Two data regions plus a small third location make an odd number of votes, so losing any one location still leaves a majority that can elect a leader and commit.

Spanner multi-region configurations do this: read-write replicas in two regions, a witness replica in a third. Witnesses vote on commits and leader elections but hold no full copy, serve no reads and never lead; Google says they make write quorums easier to reach without full-replica cost. A MongoDB arbiter is the same idea: one vote, no data, never primary.

MongoDB warns against running an arbiter on a host that also runs a data-bearing member; by the same logic, a witness sharing power or network with one region adds a vote but no independence. MongoDB also notes that arbiters trade redundancy for cost, and after one data region is lost only one copy remains.

A witness supplies a majority, not better knowledge. Survivors may act whether the other region is dead or only cut off, which two regions alone cannot do. It sits in every election, so it belongs in The dependency you did not list.