So there are two Type C ports on the X, one marked “main”. Cable testing only works with the non-main port, so what in the world is the “main” port for? Is it for cables that have a Type C connector at the base end instead of a Type A?
Yes, your understanding is correct. The two Type-C ports are for the cables with both Type-C ends. If your cable has one Type-C end and one Type-A end then you need to use the Type-C port on the left hand.
“Main” means it’s physically connected with the Type-AF port. If you look into the detail, the way of marking these two ports is different with the other ports.