In some cases, multiple sets of records may use identical join keys, or a single record may include multiple keys (such as a database table with two Id columns). These sections cover how joins are handled for such situations.
About tweaking left joins
The Multi Sub-records setting (on the Record Assembler editorRecord Join tab) changes the behavior of a left join if a record from the left source has multiple values for the join key. It is used only used with left joins. Enabling this option forces Forge to create multiple keys for such records.