This PmTcaUnsigned64 is the reference value against which the Adjustment TCA's upper and lower boundaries are computed, per its DESCRIPTION, and critically it is not something an operator sets -- it is supplied to the PM feature by whatever component actually manages the monitored entity (for example, the interface's own rate-negotiation logic). Because this basis is supplied externally rather than configured, it can itself change over the life of the port (for example after an autonegotiation renegotiation or a bandwidth profile change), which is exactly why the threshold objects are expressed as offsets or percentages relative to this value rather than as fixed absolute numbers. For example, on a Basic Tx instance this basis might track the interface's negotiated line rate, so the same fixed AdjustmentThreshold in bytes/sec means something different in practice before and after a link speed upgrade.