This read-only integer identifies which MST instance a per-port configuration row belongs to, since a single port can have separate path cost and priority settings for each instance it participates in. An admin uses it to make sure they're viewing or editing the settings for the correct instance rather than the default CIST. For example, a row with instance ID 1 holds this port's path cost specific to MST instance 1, which can differ from its settings under instance 0.