This wiki is centered around the SCP foundation yet can't be bothered to give a solid and unyielding baseline for what it is.

It does give definitions for various concepts though? https://scp-wiki.wikidot.com/glossary-of-terms

D Class — Foundation employees deemed expendable if necessary for testing or containment. Typically consist of prison inmates recruited by the Foundation for this purpose in exchange for reductions in their sentence, although Foundation employees may be demoted to D Class for egregious violations of protocol, and it has also been claimed that D Class are artificially created

(Nvm you're right these definitions are more descriptive than prescriptive)