What is the use of the second port of ZeroTier

Hello, I was wondering what ports does ZeroTier need to work properly?

Indeed, I saw this in the documentation:

  • 9993
  • Secondary Port, randomized each start up and after being “offline” for too long.
  • Random Port for UPnP

But what is the second port for? Just if the first one doesn’t work?

Can several nodes connect at the same time on the same port of a node for example can 192.168.0.1 and 192.168.0.2 connect simultaneously on 192.168.0.3:9993? If so, what is the second port for?

Indeed, I ask this because by doing sudo zerotier-cli peers I see some nodes in 9993 and others on random ports. I would like to be able to create firewall rules in input (on my LAN interface) and therefore I was wondering which ports should be opened. Just 9993 or the second is needed?

Thank you