Plural noun + noun

Dear professional teachers and native speakers,

I often get confused when I look at the noun like "Software Requirements Specification" or any other nouns which consist of "plural noun + noun."

Is this form grammatically correct?
Does not the plural part (= Requirements, in this case) have to be changed to a singular form (= Requirement) instead?

I learned that when a noun is used as a modifier, it must be in its singular form though...

Someone, please help me solve this problem