It is the terminology that AU 6 uses. It says "Setup AirPort Extreme to Extend 'SmithWireless' over Ethernet" They made it so people could easily understand it and it also allows the roaming network to host the guest network.
Both ways accomplish the same thing, but AirPort Utility 6 is easier. It also sets up the guest network to allow roaming. The second base station is not connected wirelessly but rather Ethernet.
Could you please post a reference or screen shot? Can't find that anywhere. I don't have another extreme to test it out with, but these are the only options I get on my Extreme.

And that extend option shown is intended for extending a wireless network between two devices that are not link by ethernet cable.
However, searching on setting up a "roaming network" using Airport devices, yields plenty of information as well as instructions.