You can use this to manually choose a UDP port that will be used on Parsec, useful in case you want to manually forward the port on your router. You can add these in the advanced settings or change it on your networking settings tab on the app. Make sure to use a value between 1025 and 65534. You can otherwise leave it at 0 for a pseudorandom number (default).
For tweaking the port when you are connecting to someone, use the line below, and forward that same port on the router.
network_client_start_port=9000
For tweaking the port when you're going to host, use the line below. If you for example use port 8000 as the starting port, if 4 people were to connect to this host, then 4 ports starting from 8000 would be used (in this case, 8000 to 8003). Make sure to forward all those additional ports if you expect multiple people to join.
network_server_start_port=9000