This read-write integer (1-15) is the exponent form of the maximum contention window that non-AP QSTAs use for a given Access Category when their backoff counter grows after collisions. It caps how far the client-side backoff can escalate, bounding worst-case channel-access delay for that traffic class. An admin might raise this for best-effort traffic so it backs off further under congestion, leaving more airtime for real-time classes.