STP prio nbr value
The prio.nbr value in the STP table is comprised of two sections. The first is an 8-bit value that specifies the priority of the port, while the second is an 8-bit number that identifies the port of the device. This number is sequential and doesn't correspond to the normal format of GigabitEthernet 0/0/1 for example.
Together as a 16 bit value, they are called a port identifier. The higher the value, the higher the port forwarding preference over lower priority ports, in the event that there are several ports linking to the same switch.
In IEEE 802.1t, this port priority format has been changed to 4 bits for the priority and 12 for the port identifier. More info on this is found in the link below.