What is "Dynamic Port Enabled" on my DURAFON-SIP used for?

The “Dynamic Port Enable” is used for multiple registration. When the IP-PBX supports this multiple registration feature, when one account receives an incoming call, two or more extensions will ring simultaneously. When you enable “Dynamic Port Enable”, each SIP account has its own port numbers, so as to distinguish these extensions.

For example:
The same SIP account is specified on HS10 and HS11 for multiple registration. When “Dynamic Port Enable” is checked, the firmware will assign HS10 to port 5060 and HS11 to port 5062 automatically, according to your setting on VoIP->Basic->SIP server port = 5060. Once one invites this account, HS10 and HS11 will ring at the same time.

http://community.engeniustech.com/topic/1146-what-is-dynamic-port-enabled-on-my-durafon-sip-used-for/