I have no such menu or setting. The only similar one is "Ethernet".
What app or software is this in?

Do you mean you have the 271 connected directly to an Ethernet port on your PC and this is the setting for that port on the PC?

Main: Marantz Cinema 50, Meridian 271, DSP7200.2, DSP5200HC, DSP5000 rears, Meridian 210.
PC/Headphones: Mytek Liberty II, Sansui CA-F1, Sansui B-2301, Sennheiser HD800, Kef R3