When I import new lines and post:
Scenario no. 1. Payment journal batch No. series defined, doesn't have Posting no. series defined.
Outcome: When lines are imported into the journal the LAST NO. USED for the No. series is incremented by number of rows imported (the imported file doesn't have a column for predefined document numbers, it uses the .). When those rows are posted LAST NO. USED for the No. series increments by the same number of rows. Meaning, the No. series incremented by twice as much than needed.
Scenario no. 2. Payment journal batch No. series and Posting no. series defined.
Outcome: When lines are imported into the journal the LAST NO. USED for the No. series is incremented by number of rows imported. When those rows are posted LAST NO. USED for the No. series increments by the same number of rows. Meaning, the No. series incremented by twice as much than needed. LAST NO. USED for the Posting no series is incremented only by the number of rows posted (which is good).