This PmTcaUnsigned64 is the highest sample-interval reading recorded during the current run of overflow-qualifying samples, per its DESCRIPTION, and it resets to zero at the start of each new adjustment interval. Comparing this value against OverflowBoundaryValue tells you not just that an overflow is happening but by how much the reading is exceeding the ceiling, which is useful for judging severity -- a watermark barely above the boundary is a marginal overflow, while one far above it is a clear spike. For example, an OverflowBoundaryValue of 600,000 with this watermark reading 950,000 shows the current overflow condition is far more severe than one where the watermark reads 605,000.