The checkpoint with a save operation can be taken with either a read lock or an access lock. If a read lock is used, the Teradata Database suspends update activity for all data tables that might write change images to the journal table for which checkpoints are being set. This action provides a clean point on the journal.
When a save operation is performed using an access lock, the Teradata Database takes all transactions that have written change images to the journal (which have not been committed) and treats them as though they started after the checkpoint was written.
Since you do not know how the Teradata Database treats particular transactions, a checkpoint with a save operation under an access lock is useful only for coordinating rollforward activities first from the restored journal and then from the current journal.