Sometimes people want to be able to access the VPN clients directly from the network behind the Access Server. So if for example you have your Access Server installed in your company network, and you have the need to connect directly to a VPN client that is connected from outside to your OpenVPN Access Server, from your company network, then this is possible. But by default a lot of setups use the NAT mechanism, which lets traffic flow from VPN clients to the private networks behind the Access Server, but not the other way around. To lift this restriction you must use routing. And when you use routing, the private network in your company network must then be made aware of where the VPN client subnet is, where it can be reached. It needs to know the gateway system that can lead to the VPN client subnet. And that gateway address then is the IP address of your local Access Server installation in your company network.
Check the reach OpenVPN clients directly from a private network page for more information.