Ports can be a bit confusing with UDP because there's two ports for each device you use. One for "sending to" (Device Port) and one for "listening" (Show Controller Source Port). These are often the same, but they can be different.
The V16Pro automatically "listens" to port 2638 at all times and automatically sends back to the port it received a message on.
Say I had a PC with "sending" destination port 2638 and "listening" source port 2000.
Say this PC sends to V16Pro on 2638.
(PC source 2000 --> V16 2638)
So, the V16Pro "gets" a message on 2638 (because it's listening), it will automatically send an "R" back to port it received it on.
(V16Pro source 2638 --> Received 2000)
Now, say you setup another port on the V16Pro with "listening" (source port) of 1999. Say it has a "send to" (destination) port of 2000.
Now, you send a "message out" from this port. It would look like:
(V16Pro source 1999 --> destination PC 2000).
If you put in a "0" into the "Show Controller" source port, it will automatically select a random port number.