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1. To add something on to something else. A noun or pronoun can be used between "tag" or "on." Excuse me, is it all right if I tag a side of bacon onto my order? If you kids keep talking back there, I'll tag another chapter onto your homework for the weekend.
2. To tap a ticket with an RFD device inside it against a digital receiver in order to pay for one's fare or indicate the beginning of one's journey. You don't have to present your ticket to the driver, just tag on as you get on the bus. Any customers who are caught not tagging on will face a $200 penalty.
To add or append something: She tagged an extra paragraph on the letter. Before sending the package, he tagged on a little note.