in-flight transactions are those transactions that have been committed (or hardened, or made Durable) on an originating instance (or a propagating instance) but not yet transmitted to a replicating instance.

An originating instance is an instance that processes business logic and generates logical updates to the database.

A replicating instance is an instance that does not execute business logic or generate database updates. It receives logical database updates from an originating instance or another replicating instance and applies them to its database.