Not from SF, but usually means "Available on Radio.." meaning that they are not in their station, or in their apparatus (near their mobile data terminal/mobile computer). For example, crew is on a handheld radio at the supermarket or getting lunch... this tells the dispatcher that they may not be able to push their "en route" button on their computer right away if they get a call, and that the crew will verbally acknowledge the call, then push their en route button once back in their ambulance.
