This field is part of the PCIeFatalStats table's tally of ACS (Access Control Services) violation errors on this compute node's PCIe fabric — ACS is the PCIe feature that isolates and redirects peer-to-peer traffic between devices, and a violation means some device attempted a peer-to-peer transfer that broke that isolation policy. This is the historical high-water mark: the highest count of this error type ever recorded in a single 15-minute window within the retention history UCS Manager keeps for this compute node's PCIe fatal-error stats, so a quiet current reading doesn't hide an earlier 15-minute burst. For example, if this counter shows 5 while the live 15Min bucket currently reads 0, it means five such errors (e.g. a non-compliant mezzanine card attempting direct peer-to-peer DMA that the platform's ACS policy blocks and flags) occurred together at some point in an earlier 15-minute window, even though none are happening right now.