When you add an armored kilt to a suit of light armor, the set counts as medium armor. Likewise, a kilt and medium armor counts as heavy armor. Adding an armored kilt to heavy armor has no effect.
The armored kilt is made of a thick cloth skirt with bars of steel hanging down from the waist and a ring of horizontal steel plates just above the hem. An armored kilt can be worn separately as light armor, or it can be added to other suits of light or medium armor. Adding an armored kilt increases a suit of armor’s armor bonus by +1, but it adds 15 pounds to the armor, lowers the maximum Dex bonus by 1, and increases the armor’s weight category (from light to medium and from medium to heavy). Adding an armored kilt to heavy armor does not provide an armor bonus increase.