This read-only value reports the slot number of the corresponding DS1 interface, scoped to a specific historical 15-minute interval entry in the dsx1Interval* table (as opposed to the 'current', still-accumulating interval). This lets an admin identify which physical card a given row of historical interval data belongs to when scanning the 96-interval (24-hour) history table across multiple DS1 cards. An engineer pulling historical error data for a specific card to correlate against a known outage window would use dsx1IntervalSlotId to filter the interval table to just that card's rows.