This INTEGER (none, terminal, or facility, default none) selects where the loopback point sits on a 10G WAN PHY client port framed as SONET when loopback is enabled: terminal loops the signal back toward the client/system side, while facility loops it back toward the line/network side. An administrator picks between the two to isolate which segment of the path is under test, since a terminal loopback exercises the client-facing electronics while a facility loopback exercises the line-facing path. For instance, selecting 'facility' on 10G WAN SONET sends the received line signal straight back out onto the line, which is the standard test for confirming line-side transport before moving on to troubleshoot the client interface.