What does FS mean in shoes?

Excuse me, I'm a bit confused here. Could you please clarify what "FS" stands for when it's used in the context of shoes? Is it a specific brand, a type of material, or perhaps a particular design element? I'm trying to get a better understanding of this term, as I'm not familiar with it in relation to footwear. Could you please elaborate or provide some context so I can grasp the meaning of "FS" in shoes? Thank you in advance for your help.