Units readied but not responding

What I mean by the title is, a unit has been dispatched to a call but it stays parked at the station just in case the call it's been dispatched to gets worse when the other unit arrives
EX:
You get a call for a burning car and you dispatch one fire engine, then you tell another unit to get ready to respond just in case it becomes a garage fire but that unit doesn't leave the station until it's needed or not needed