For UCS-Express support, please post your questions here: http://developer.cisco.com/web/srev/forums . To log into the site you'll need a Cisco Care Online (CCO) ID, which you can create free of charge by clicking on the 'Register Now' link.
Regarding your question, the External Interface Configuration refers to the external port located on the SRE module. This port configured via the vSphere Client, would allow packets to bypass the router as shown in Figure 3-7 in that document.
Regarding the difference between the External Interface and the MGF Layer 2 Switch Configuration, the switch configuration still utilizes the router for communication, but bypasses cpu utlization which in effect adds less load to the router. Also EtherSwitches have many ports, and are directly configurable as you know while the External Interface has just one port.