This read-write value, from 1 to 200000000, assigns the path cost this port uses within a specific MST instance's spanning-tree calculation, separate from the port's cost under any other instance. An admin can give the same physical port different costs across instances to steer different groups of VLANs onto different physical uplinks. For example, giving port 3 a lower cost specifically within instance 1 lets the VLANs mapped there prefer that uplink, while VLANs in instance 2 might still prefer a different port.