This PmTcaUnsigned64 is the basis value minus the configured underflow threshold, per its DESCRIPTION, and a statistic reading at or below this boundary at the end of a sample interval registers an underflow condition, incrementing the underflow current count toward the configured UnderflowCount. Just like OverflowBoundaryValue but on the low side, this boundary is evaluated every sample interval rather than once per Adjustment Interval, which is what lets underflow detection catch a service collapsing toward zero well before the slower main Adjustment mechanism would notice. For example, with a basis of 900,000 bytes/sec and an underflow threshold of 400,000, this boundary sits at 500,000; a sample interval averaging only 320,000 bytes/sec falls below it and registers as an underflow condition.