Ok, normally when people speak of "light kilt weight" they mean anything in the 10 oz to 12 oz range. The weight cloth is generally used for lady's skirts, light weight men's kilts, and can also be made into ties, scarves, sashes, vests, trousers, etc. Lots of uses, which is why many of the mills have a much wider range of tartans available in this weight. If they are only going to weave a tartan in one weight range, it will likely be this one because the cloth has so many applications.

When people speak of "medium weight" they typically mean a 13 oz to 14 oz cloth. This cloth is used for men's kilts, as well as heavier weight women's skirts, but also suitable for uphostery and the like.

Heavy weight is normally 16 oz per yard. I would class 15 oz cloth as heavy, but I can see where someone else would class it as medium. This is the best cloth for making men's kilts and is usually woven specifically for that purpose.

Regimental weights can be even heavier, anything from 18 oz to 22 oz per linear yard.

Sometimes you will also see what is called "spring weight" which is typically about 8 or 9 oz, and often a plain weave (as opposed to a twill). This is used to make ties, scarves, sashes, and the like, but not made up into skirts or kilts.

Aye,
Matt