What is a "scan request" in the context of Availability Groups?

The documentation for [sys.dm_hadr_database_replica_states](https://learn.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-hadr-database-replica-states-transact-sql?view=azuresqldb-current) states > 1 = Synchronizing. For a primary database, indicates that the database is ready to accept a **scan request** from a secondary database. For a secondary database, indicates that active data movement is occurring for the database. This is the first time that I've seen the phrase "scan request" in the context of Availability Groups. I cannot guess what it means nor find it elsewhere in the documentation. What is it? Where is it documented?