How do you decide on "well known sources"

To avoid plagiarism, a CFA charterholder/candidate is supposed to cite all references unless they are from well-known sources. My question is, how do you know when NOT to cite?

Are all US government agencies (maybe even obscure ones) well-known sources?

In Perl case C (reading 5), Sherman uses the International S&P Stock Guide. Is this a “well-known” source that he need not cite? How about the Bloomberg service? Or, are all private entities “not well-known” and must be cited?

If you can point to any clear guidance _ from CFAI _ I’d appreciate that. (Must be from the CFA Institute, not your gut feel.)