The two guards
Double-booking checks a new assignment against the driver's other jobs, padding each window by the buffer minutes you set. Off-duty checks the driver's duty status at the time of the job.
Modes
Off: the guard never surfaces. Warn: the conflict is shown in the assign drawer and the dispatcher must acknowledge it with a reason, which is audited; the server does not reject. Block: the server rejects the assignment unless it is overridden with a reason, which is audited. Dispatch → Settings → Assignment guards.