The OpenVPN tab is, under the hood, just an interface group containing all OpenVPN instances - all servers and all clients. You can use it to generally control traffic into your firewall from OpenVPN. You cannot, however, get special things like reply-to, which automatically sends reply traffic back out the interface into which it arrived because it is not an interface, but a group.
If you assign an interface to an OpenVPN server or client, the rules there apply ONLY to that server or client and you get magic things like reply-to. You can also use it to perform outbound NAT, policy route to it (because the assigned interface has a matching gateway), etc.
If you want to take advantage of this, the rules on the OpenVPN tab must NOT match the traffic you are interested in because they are processed first and first match controls.
I generally delete all rules on the OpenVPN tab when I start using assigned interfaces.
If you want more information I suggest a gold membership and the included OpenVPN hangouts and pfSense book.