Used only with primary key match rules, Key Match jobs run the match process on records from two or more source systems when those sources use the same primary key values.
Key Match jobs compare new records to each other and to existing records, and then identify potential matches based on the comparison of source record keys (as defined by the primary key match rules).
A Key Match job is automatically created after a primary key match rule for a base object has been created or changed in the Schema Manager (Match / Merge Setup configuration).