What is the differences between the DID server and the other 2 port server

The DID model FF110 connects to a DID line (2 wire battery supervised loop start system) for inbound faxes and an has an analog modem for outbound faxing. The standard two port model FF200 has two analog modems for connecting to PSTN or PBX station ports....These modems are used for both inbound and outbound faxing. Here are the spec sheets: