This counts RADIUS packets received on the authentication UDP port from this server whose packet type code the switch doesn't recognize. A nonzero value here is a specific signal of a RADIUS protocol version or vendor-extension mismatch, for instance a server sending a newer packet type the switch's firmware predates, rather than a network or credentials issue, since the packet arrived and parsed enough to be identified as unknown type rather than simply dropped. For example, if a RADIUS vendor pushes a firmware update that starts sending a CoA (Change of Authorization) packet type the switch doesn't support, this counter would begin incrementing even though authentication itself keeps working.