Dave,
After I saw the mark on the tray, I opened the manual.
I'm usually one to obey the manuals. But there are some things I don't want to just place on the tray, the way the manual says. So while I won't let any foil contact the tray directly, I'm going to use a plate to keep anything I feel a need to wrap, (it was a baking potato,) in foil away from that metal tray if I don't just use the "Convection" function.
I once had an old nuker I wanted to get rid of, and was so tempted to put foil in it, just to watch the sparks.
Steve