Ontario hosts who need the letter sworn will usually go to a lawyer or licensed paralegal, both of whom are commissioners for taking affidavits by virtue of office under the Commissioners for Taking Affidavits Act, R.S.O. 1990, c. C.17. A notary public appointed under the Notaries Act, R.S.O. 1990, c. N.6, can also notarize the signature. The practical difference matters: commissioning attests that the deponent swore the contents, while notarization attests to the identity of the signatory and the authenticity of the signature. Visa offices that ask for a "notarized" letter generally accept either, but the safer route is a notarial certificate.
British Columbia is the outlier, because it maintains a separate profession of notaries public regulated under the Notaries Act, R.S.B.C. 1996, c. 334. BC notaries handle invitation letters as routine work and their seals are recognised abroad without difficulty. Alberta consolidated both functions in the Notaries and Commissioners Act, S.A. 2013, c. N-5.5, so an Alberta host will often find that a single appointment covers both the oath and the seal.
Quebec hosts are on identical federal footing, since immigration selection for temporary residents remains a federal matter and the Certificat d'acceptation du Québec applies to permanent and certain long-stay categories rather than ordinary visitors. Where a Quebec host wants a sworn version, the letter goes before a notary who is a member of the Chambre des notaires du Québec or before a commissioner for oaths, and the resulting notarial act carries strong evidentiary weight. Hosts elsewhere who need the same formality can pair the invitation with a sworn affidavit carrying a properly drafted jurat.
Visa office practice is the variable nobody controls. Offices covering India, Nigeria, Pakistan and the Philippines apply visible scrutiny to family visit files and frequently expect notarization, while several European offices never mention it. Check the visa office instructions for the applicant's country before deciding whether to notarize, because sending an unnotarized letter to an office that expects one wastes a full processing cycle.