What is "fusable interfacing" and who uses it to make kilts?

[Heming]

In a traditional kilt, the waistband is about the only place I could see it being used. (maybe around the slit for the under apron strap to pass through. ).

That is actually quite a good idea that I wish I had thought of before attatching the waistband to my lightweight PV kilts.

It probably won't be necessary on heavy wool, though.