This PmStatBitmap identifies which of the watched statistic(s) actually established the current overflow high watermark value, per its DESCRIPTION, and depending on PmTcaScheme this may be the same statistics being watched overall or only a subset of them. Exactly like the main HighWatermarkStats object but scoped to the overflow condition specifically, this tells you which individual counter is driving an in-progress overflow, which matters when a profile watches several statistics at once and you need to know which one to actually go investigate. For example, if this bitmap identifies TxBytesPerSec rather than TxFramesPerSec on a Basic Tx instance, the overflow is being driven by raw byte volume rather than an unusual number of very small frames.