Was at bars yesterday, what is that receipt you get for?

Went to Moxies for example and they added a 20% service charge to everything. Yet I always got a receipt (despite entering the PIN code) which had a tip line on it. Are they expecting a tip in addition to their service charge? Also if I chose to add a tip how would that work if they already ran my card? Like I write how much to tip and the total and then how do they get the money? Run my card again?