This read-only value reports the port number of the corresponding DS1 interface on the board, scoped to the historical interval table, pairing with dsx1IntervalSlotId to fully identify which physical port each historical 15-minute interval row describes. This is the port-level identification needed to interpret the interval history table correctly when a card has multiple DS1 ports. An engineer building a per-port historical error report would use dsx1IntervalPortId to separate the 96-interval history into distinct series for each port on a multi-port card.