What is requests first and second priority in job priority

The NBU admin (vol I) guide says:

NBRB evaluates the requests sequentially and sorts them based on the following criteria:

■ The request's first priority.
■ The request’s second priority.
■ The birth time (when the Resource Broker receives the request).

The first priority is weighted more heavily than the second priority, and the second
priority is weighted more heavily than the birth time.

Whats does it mean by request's first and second priority.

Thanks

Mike