Played with both boys and girls, when you are "it", to tag someone you come up behind them and give him/her a swift slap right on the bum cheek, or you can slap both cheeks, if it pleases you to do so.
Dood, we got a whole gang togther and went out in a field and played a nice, friendly game of ass tag!!
EX. 1: have you seen Maria's ass tag? it's a sea cucumber
EX. 2: i was so fucked up last night and i hooked up with a chick, and i have no fucking clue what she looks like. all i fucking remember is her ass tag, it was a zombie unicorn.
The accepted standard that the tagged side of a towel is for designated nether-regions while the non-tagged side is reserved for the face and hair. This convention is used to avoid cross-contamination and is to be assumed as general practice. Practical in situations where one must use a borrowed or foreign towel.
"I forgot my towel in the shower and had to use my roommates. Hopefully he observes the ass-tag convention."