Two different offices
A commissioner for taking affidavits can administer oaths, affirmations and declarations — the sworn-statement side. A notary public has broader authority, including witnessing and executing certain documents, certifying true copies, and issuing notarial certificates often used abroad. The authorities come from different statutes.
Where people go wrong
The common error is treating a commissioner's authority as if it included notarial acts. Certifying a copy as a true copy, or issuing a notarial certificate for a foreign authority, is a notarial act — not something a commissioner-only appointment covers. Know which office you hold for the act in front of you.
Ontario lawyers and paralegals
Every practising Ontario lawyer is a commissioner for taking affidavits by virtue of office, and lawyers are commonly appointed as notaries public. Licensed paralegals' authority is narrower and defined by their licence. Confirm your own current authority rather than assuming it from your title.
When to decline or refer
If a document needs an act you are not authorized to perform, or needs legal advice rather than commissioning, decline that part and refer. Doing the act anyway, or blurring commissioning into advice, is where complaints start.