It's both actually.

They shout out your number as you're running through transition and hopefully one of the volunteers will hear it, grab your bag and give it to you. It certainly helps to remember what row your bag is in.

I'd tie your favorite color ribbon to your bag so you can easily identify it.....

Double check your Trans bag on race morning to make sure everything is in there and its in the right place.

I second Smitty. Just a matter of if someone hears your number or not. Usually they try to con kids into grabbing the bags so make sure it is your when you get it as well! I was handed the wrong bag in CdA last year so check it!