It's kind of the currency of the train. You seem to get more at certain times, usually around when arriving at the platforms. Not sure what prompts getting certain amounts though.

As far I know you can ask for anything. I'm sure the train will stop you if you try to get something that's too much. Like, I doubt the train would allow my request for my Gundam... but given its sense of humor, it might allow me a tiny version as a statue. Also, be careful requesting a random set of clothes. It's a preset option. Mine aren't that bad, but I've seen some strange ones.