On the six buttons models, to connecte the expansion module. Also some card reader or other device has been known to be connected, but that is not common.
I think there's a guide to build a cable somewhere on the net with this cable you can access the phone and set up some network parameters in case it is not operational and not accessible with normal things like CCM or it getÃs no IP address.
HTH
Chris
Getting Started
Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: