This PmTcaUnsigned64 is a frozen snapshot of whichever specific boundary -- UpperBoundaryValue or LowerBoundaryValue, per its DESCRIPTION -- was actually reached, exceeded, or preceded to cause the most recent end-of-interval Adjustment TCA. This is the single number that tells you, after the fact, exactly how far the interval's high watermark had to travel to trigger that historical event, letting you judge in hindsight whether it was a narrow, marginal crossing or a dramatic overshoot. For example, a recorded boundary of 600,000 next to a LastIntvlEndTcaBasisValue snapshot of 500,000 (with a threshold of 100,000) tells you that TCA was triggered by the watermark reaching the ceiling exactly, a marginal crossing rather than a dramatic spike far past it.