At time of writing, using invRoughness/metalness protocols to match folks current workflow of 'want sharper highlights, ramp the numbers up'. Been some back and forth on whether to just join the industry standard and do rough/metal, but figure that's a last minute cleanup detail, really. Particularly since we've got a switch there for swapping between modes.
Also at time of writing to save reference tracking space/speed, using what we're calling a composite map protocol that will either ramload smooth|ao|metal singulars into a single internal bitmap, or straight up load one with the relevant RGB channels already packed (also with a nice shiny 'save the composite' button for packing the three singulars into the one, though at present that does at least require all 3 bitmaps to share the same resolution. )
Or's this more a 'what spec method' type? if so, GGX since that was what everyone and their dog asked for when we started this.