This PmStatBitmap identifies which of the watched statistic(s) actually established the current underflow high watermark value, per its DESCRIPTION, with the same same-set-or-subset relationship to PmTcaScheme as the other watermark-stats objects. This is the underflow-specific version of HighWatermarkStats, telling you which individual counter is actually behind an in-progress underflow when a profile is watching more than one statistic on the same PM instance. For example, if this identifies Esl rather than Sesl on a DS3 circuit, the underflow condition (an unexpectedly low error-second count, in a context where low was defined as abnormal for that particular monitored quantity) is being driven by the plain errored-seconds counter specifically.