This PmTcaUnsigned64 is the basis value minus the configured adjustment threshold, per its DESCRIPTION, and a statistic reading at or below this number is a candidate to trigger an Adjustment TCA once the current adjustment interval ends. This is the floor half of the same normal band whose ceiling is UpperBoundaryValue, so together the two objects are what you'd graph as a shaded 'normal' corridor around the basis value on a dashboard, with anything the reading strays outside of during the interval flagged for the end-of-interval TCA evaluation. For example, with the same basis of 500,000 and threshold of 100,000, this boundary reads 400,000, so a sample dropping to 350,000 is a genuine undershoot candidate for an Adjustment TCA.