Define Errors, Warnings & Notices

Would be really valuable to know how htmlcs defines errors, warnings and notices. I couldn't find any documentation on this. Is there a list of rules or something? I assume errors are things the code is sure about, warnings need human verification and notices are ??

This is good http://squizlabs.github.io/HTML_CodeSniffer/Standards/WCAG2/

But would like to know how they are categorized.