A SIN is how the GSA Schedule breaks its one consolidated contract into specific categories, so a buyer looking for IT staffing and one looking for office furniture are searching under completely different SINs even though both are technically on the same Schedule. Your Schedule contract lists the specific SIN or SINs you are approved to sell under, and that is what determines which eBuy requests you are even eligible to see and quote on. Holding the Schedule itself does not help if the work being requested falls outside your awarded SINs.

In a sentence

"Quotes are being requested from Schedule holders with an active SIN in this category." If your Schedule contract does not carry that SIN, you are not eligible to respond, no matter how well your business could actually do the work.