Path definitions in I/O Setup determine how audio is routed through, and metered on, your audio interfaces.
Pro Tools internal (on-screen) metering of 5.1 format paths always follows the Film track layout:
Pro Tools track layout of 5.1 format audio tracks in the Edit window also conforms to the Film layout (arranged top to bottom). See
5.1 Track Layouts, Routing, and Metering
In the I/O Setup dialog, you can route these signals out of your audio interfaces according to any track layout (such as DTS or SMPTE/ITU).