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

[Dixiecat]

Interfacings are using in making garments to give the fabric body, to strengthen weak fabrics and/or to give reinforcement to parts that need support such as buttonholes.
Interfacings are used under the main fabric, usually applied to a facing, but can be applied directly to the main fabric.
Interfacings can be made of any fabric, but usually (these days anyway) are made from a non-woven fabric. Bespoke tailoring will most likely use sew-in interfacing rather than fusibles.
Fusible interfacings have a heat sensitive resin on the wrong side so that you can apply the interfacing to the garment fabric using an iron.

[vorpallemur]

Interfacings are using in making garments to give the fabric body, to strengthen weak fabrics and/or to give reinforcement to parts that need support such as buttonholes.
Interfacings are used under the main fabric, usually applied to a facing, but can be applied directly to the main fabric.
Interfacings can be made of any fabric, but usually (these days anyway) are made from a non-woven fabric. Bespoke tailoring will most likely use sew-in interfacing rather than fusibles.
Fusible interfacings have a heat sensitive resin on the wrong side so that you can apply the interfacing to the garment fabric using an iron.

There are three sorts of interfacing construction. There are non-wovens, which are mostly needle-punch felted. these tend to be stable in all directions, which means no stretch. there are some all-bias (stretch equally in all directions) variants around. There are woven interfacings, which are usually stable warp (length) wise and weft (width) wise, but strech on the bias. There are knit (usually, but not exclusively tricot) interfacings. They've got various stretch properties, usually they're stable one way and strech the other. They're usually more flexible, so they drape better. There are weft and warp insertion interfacings, which are knit, but then have a weft or warp thread inserted into them. Less stretch, and less flex.

[Dixiecat]

"Stitch Witchery"

This is not an interfacing. Stitch Witchery is what's called fusible web and used as you described.

regardless of Dixiecat's points (because in time the fusible does leave a dimpled surface after time and sewn in doesn't)

I can only assume that you've used low-quality fusibles and/or applied them incorrectly and/or used the wrong fusible for a project. I have made quite a few garments using these products and have had no problem with them when used properly. I'm totally serious.

And yes, the answer really is, don't use fusibles in kiltmaking!! The best product is the hair canvas.