It's a unique ID for all of your shipments scheduled to be picked up in a day by a single courier partner. Simply click on the pickup ID to view your shipments.
Case 1: Five shipments with the same courier partner are scheduled for the following day: In this case, a single pickup ID will be generated for all five shipments.
Case 2: Five shipments are scheduled for the following day with a different courier partner: In this case, a unique pickup ID will be generated for each shipment.