FS stands for "Factory Seconds", which are boots that have slight cosmetic imperfections or minor manufacturer defects that do not affect the performance of the boot.
Clearance are boots that have been discontinued in North America.
Both Clearance and Factory Second boots are sold as-is, without a warranty. They are not eligible for warranty repair or replacement under any circumstances. They can be returned for a refund of the purchase price only within 30 days of purchase as long as they remain in unworn condition.
Both Clearance and Factory Seconds are NFPA/ASTM/CAN/CSA certified at the time of manufacture. Some models may be certified to an earlier standard.