By default, all traffic is marked as having a Normal priority level. Traffic shaping rules that are marked at the same priority level share the same fraction of their respective levels. For instance, if there are 5 traffic shaping rules marked as High priority on a 10Mbps pipe each rule would have access to ~1.1Mbps.
Each traffic rule supersedes each rule below it and the rules below it must strictly adhere to their fractional bandwidth limits. For instance, if there is a high traffic shaping rule but no low traffic shaping rules configured then the high priority traffic would have access to 5/7 of the available bandwidth on the uplink and normal traffic would have 2/7s. Additionally, if there are no high priority traffic shaping rules then normal priority traffic gets 6/7 of the bandwidth and low priority gets 1/7 of the uplink's bandwidth.