Note that not all airlines have the same codes even! Some of the codes given for 'discounted' fares are even for "award" ticket codes.
You need to 1) check with each airline individually, or 2) get your travel agent to check the best fare for you.
Y for example seems like it's full fare economy. But on airlines like Singapore Airlines, Y is used for a variety of fares in economy class AND full fare economy! Likewise on British Airways, they use "S" for their "Y" fares - ie Full fare economy. Whereas "S" is the discounted Business Class fares for SAS.