The desc explicitly states this is the count of IPsec UDP ESP encrypted packets — ESP packets encrypted specifically for UDP-encapsulated ESP, the NAT-Traversal mode where ESP is wrapped in UDP, typically port 4500, so it can pass through NAT devices. A nonzero, growing value here specifically indicates at least one VPN peer is behind a NAT device and therefore using NAT-T rather than native ESP, useful to know because NAT-T tunnels have a different troubleshooting path, UDP/4500 reachability, than plain ESP tunnels. e.g. cpvIpsecUdpEspEncPkts climbing steadily for a remote-access VPN user connecting from behind a home NAT router, while a direct site-to-site tunnel between two public IPs shows growth in the plain cpvIpsecEspEncPkts counter instead.