Notarial wording — whether preprinted on a document or as loose certificates — often lists a signer's title, such as "John Smith, President of ABC Inc." This designation is called the signer's "representative capacity," and notarizations with this designation need to be handled with care. That wording on an acknowledgment or jurat means you are "certifying" that John Smith is the President of ABC Inc., and you have no way of verifying such representative capacity.
In California, for example, Notaries are prohibited from certifying capacity. If the wording of an out-of-state certificate includes the signer's capacity, replace it with a loose certificate that does not mention the client's title or role.
In other states, when you encounter paperwork that lists capacity in a company and your signer doesn't have proof of that, you still can notarize for the person as long as the notarial language in the certificate makes no such assertion.
Signers can write their name and anything else after it without the Notary requiring proof. All the Notary is doing is verifying identity. You're not vouching for or certifying capacity, and the notarial certificate will reflect that.