This read-only PerfCounter64 counts received bit errors since the most recent synchronization event, per its description, resetting each time the link resyncs unlike the SinceStart counter which never resets during the test. An administrator uses this to isolate error behavior in the current sync period specifically, which is useful right after a resync to judge whether the link has stabilized. For example, a low bertTestStreamMonRxBitErrsSinceLastSync shortly after a resync but a high SinceStart total would tell an operator most of the errors happened before the most recent resync, not after.