Clothing : Difficulty

Hello,
On the clothing, when I see a difficulty of 5-15 (example) what does it mean? Is it like some kind of level? Does it means that this piece of clothing will be usefull for the proficiencies in that level range?
On the clothing, when I see a difficulty of 5-15 (example) what does it mean? Is it like some kind of level? Does it means that this piece of clothing will be usefull for the proficiencies in that level range?