Destination Ports versus NAT Ports in Port Forwarding

[VirtuousVigor]

Hello IT Fellows,

I would like to learn from a brief explanation on the difference between the destination ports and the NAT ports in Firewall / NAT / Port forwarding on a FreeBSD 11.1-RELEASE-p7 pfSense.

Basically, there are two implementations for MS RDP port forwarding and each go to NAT port 3389 which is the default MS RDP port.

But, the destination ports differ for each. One implementation goes to 33389 and the other to 33390. Yet both use NAT port 3389.

I suppose what I am asking is this a matter of firewall ports in LAN versus NAT WAN ports? Or physical vs logical ?

I am just want to understand better. Thank you in advance.

Alexi