Avonlea,

I'll answer this question of cutouts for Jamie because he has super secret Mod Squad stuff he must attend to.

In a Traditional Kilt the area of the Fell (The area in the back of the kilt between the waistband and the hip) is tapered and the pleats are sewn down. When you taper the pleats the overlapping fabric can get quite thick. We in the Kilt world call this "The dreaded pillow butt". So the kiltmaker cuts away the excess material on the inside of the kilt to make it thinner. They then put in a canvas interfacing or reinforcing to take the strain of strapping the kilt on and the wearer moving.
When all this is done a light cotton liner is sewn on to cover the cut-away area and hide the interfacing.

On some kilts this Fell area does not have the excess fabric cut-away and this can sometimes cause the wearer to sweat in the small of the back.