When activity slips are removed from the Multiple Slip Entry view
Activity slips no longer appear in the Multiple Slip Entry view after you've purged them using the Purge Activity Slips window (step 1). If you never purge your activity slips, they may remain in the Multiple Slip view indefinitely. (To purge activity slips)
An activity slip can be purged only after it has been billed on an invoice, and that invoice has been purged. (Purging information that's no longer needed)