This confused me too a few years ago.
Answer: The RID number is the drone's serial #. It would be nice if DJI said so in the manual, but they're very coy about RID documentation for some reason.
In the future, this is a really bad idea, and is in fact illegal. Drones like any aircraft must be Inspected before flight, and can not fly if there are any problems that would interfere with safe operations. This certainly qualifies.
If that...