It is used for whatever you decide it is used for.
Do I have to decide that my nodes and application software will use a certain fport number?
That would be the general idea, yes. Start with 1 for your first packet type and extend from there as your needs grow.
If they are not the same what happenes ?
You probably might need to set a legal value for packets to be sent and correctly interpreted. 1 is a legal value.
Depending on what software you use in the node, you may need to ignore illegal values if they are delivered to you. It’s probably simpler to only act on expected values.
TL;DR set it to 1 and check for that.
Or any other legal number you like.
Uplink and downlink numbers are distinct, you can chose to make them match or not.