This read-write value, from 0 to 240 in steps of 16, influences which port on a shared LAN segment is chosen as designated (forwarding) versus blocked when path costs are otherwise tied. An admin lowers the priority number on the port that should win that tie-break. For example, setting one uplink port's priority to 0 while leaving a redundant second uplink at the default 128 makes Spanning Tree consistently prefer the first port as the forwarding path whenever both have equal cost.