Nik,

I'm just confused now so:

Tubs. Stitched-in innertube and mounted on relatively lightweight tub-specific rims. Tyres, open tubs and tubular clinchers CANNOT be mounted on these rims.

Tyres, open tubs and tubular clinchers: These have a beed of wire or Kevlar that hooks onto a corresponding hook on the wheel rim. Tubs CANNOT be mounted on such rims.

Have I got it now?