Join compares key fields from two input streams. Connect the first stream to L and the second to R, then choose the matching key on each side. J returns matching record pairs. The L output holds records from the left input without a match, and R holds records from the right input without a match. These are different outputs, not different names for one table.
A repeated key on either side can produce several matching pairs. Two left records and three right records for the same key create six joined records. Clean and type the keys deliberately before joining, and decide whether duplicates are valid at the intended grain. Use the unmatched streams as evidence when a reconciliation loses workers or invoices. A missing match should lead to an explanation, not a silently discarded record.