The art of placing stickers onto objects such as stop signs, lamp posts, crosswalk signs, and any other available signs or walls. Also placed in subway trains, transportation buses, and on places of high traffic. Placing a slap tag on any other type of graffiti (spray paint tag, throwie, piece, or mural) is considered disrespect. Slap tags are usually drawn on label 228 of the USPS mailing labels, or printed by the tagger with their personal printer on paper with an adhesive side.
Benny drew his own slap tags and slapped them all around town. A nice way to get your name up fast.
— Urban Dictionary