I am not a native English speaker. When I was at school I was taught to follow grammar rules and not to invent my own ones. I have a question about the following rule: a noun used as an attribute cannot be plural (a 3-room (not roomS) flat, a 3-day (not dayS) rest, a 3-year (not yearS) course). However now I can see a lot of examples in scientific articles where this rule is not observed. Moreover, native English speakers seem not to see any strange things about it (you can read two replies to my topic "why “sciences”. There I asked why the title “The IEEE Nuclear and Plasma ScienceS Society” was correct. The replies explained to me what that title meant. However, I still feel I don’t know when a noun used as an attribute can be plural and when it can’t.
With best wishes!.