Time Machine: What files and folders are affected when excluding System Files and Applications? When you put System Files and Applications in the exclusion list, it is not clear what specific files and folders are being concerned.

Is it just the /Applications and /System folders? 

Are /private or /opt also excluded? How about /Library?
According to Apple


  OS X system files include the System folder and the apps that come with your Mac, such as Safari, Mail, Messages, iTunes, Dashboard, and others.


I'd also like to see a complete list. Based on my own testing (on 10.11.1) I can confirm that /opt is not included and that not everything in /Applications is excluded.