Advanced > OpenVPN is remote client VPN.
You can read more about that here: Configure Remote User Access using OpenVPN
Or in the manual - https://download.peplink.com/manual/pepwave-max-user-manual-8.3.0.pdf
OpenVPN WAN is used when you want the router to access an OpenVPN service.
I don’t know how to respond to this. What did you expect to find on the website?