The op log entries of a mongod node may include an optional fromMigrate field, defining if/how their content should be consumed by downstream logic (e.g. replication machinery, change stream readers).

Originally introduced through SERVER-4333 to manage update statements generated as part of a chunk migration, its use has widely extended to mark any operation on data and catalog metadata that supports "internal activity" or "tasks on temporary namespaces"; at the same time,  the fromMigrate terminology also widely propagated throughout the codebase in related variables, API signatures and serialization schemas. This opens the way to potential code regressions, since the semantics around such a terminology are difficult to interpret and documentation is scarce.

The objective of this ticket is to refactor the codebase to reduce such a friction, while still ensuring backwards compatibility.