peer-to-peer connectivity
reason; not everyone can open a port, and/or or their isp has complications with them doing so.
how it would work; if 'p2p-enabled' in the server config is set to true, let whitelist.txt act as the stand-in file for the appropriate setup [as p2p connections have little to no security, so whitelisting user accounts makes logical sense, unless there's another method i am not aware of]
Quite sure you still require port forwarding for that
You don't need to open a port to use OpenVPN to connect two machines. If both machines are behind NAT they have to have access to each other's public IP address, for example by using dynamics DNS.
The two ends then both send out UDP packets (eg pings within the tunnel) to each other which will open the connection. The private IPs within the tunnel are then available for communication.
It's easier if the server can have an open port and static IP.