This counts Mode Config payloads this device sent that the remote client rejected, per its description - meaning this device, acting as the VPN server, offered an address or attribute the client refused to accept. It's a client-compatibility signal: if one brand of VPN client consistently rejects a specific proprietary attribute, this counter climbs only on tunnels to that client type. An admin can use it to isolate which client software needs a particular Mode Config attribute disabled at the head-end.