When Multiload (non-delete) is run straight to Teradata DBMS, locks on base multiload tables are acquired only during the APPLY phase, which starts around CHECKPOINT END. This allows users to continue accessing the base load tables until the APPLY phase starts.
Multiload (non-delete) has limitations with Unity. Unity acquires a READ lock on the base load tables at the start of a load. This supports Unity recovery of systems and tables. Client programs can continue to access base tables with read and access locks, until the load runs the CHECKPOINT END command. Queries that request or require the WRITE or EXCLUSIVE lock must wait until the load job terminates or completes.