If you set this POS option in Options Configuration to Yes, a clerk can redeem a raincheck by pressing Ctrl-P (or the Promo button). When you activate Advanced Rainchecks, you want clerks to redeem rainchecks by using the new "redeem raincheck" feature. However, clerks are usually in the habit of clicking the Promo button to redeem rainchecks because customers typically return for their raincheck item after the item has gone off promotion. With the "Have Ctrl-P Prompt for Raincheck" option set to Yes, you eliminate the problem because the clerk can properly redeem a raincheck when he or she clicks Promo (or Ctrl-P).
Set this option to No if you want clerks to only be able to redeem a raincheck by clicking Menu, and then clicking Sell Raincheck Item.