Two do the deciding work: proof of Canadian or Mexican citizenship, and the credential named in the profession's own entry on the agreement's professional list. The employer's letter frames the job but cannot supply either fact.
Match one record to each condition
Citizenship is proved by a passport or citizenship certificate, and a permanent resident card cannot stand in for it, however long the holder has lived in the province. The credential requirement is decided by the appendix entry, so read that entry literally: if it calls for a baccalaureate degree the transcript and parchment matter, and if it calls for provincial licensure the regulator's letter matters. Where a degree was earned outside Canada, ask early whether an equivalency assessment is expected and who is accepted to prepare it. The employment letter then does a narrower job than people assume. It describes the employer, the professional duties, the remuneration and the period requested, and it confirms that the arrangement is employment rather than self-employment in the United States. It cannot cure a mismatch between the role and the listed profession, and it cannot certify a credential the applicant does not hold.
Hypothetical example: A bilingual accountant with Mexican citizenship is hired by a manufacturer to reconcile inventory accounts and prepare financial reports. The initial review should compare the real duties with the accountant entry and identify the degree record or qualifying alternative that the entry calls for. A passport settles nationality; a detailed employer letter settles the offered role; neither replaces the required credential. If the work will be performed through an independently owned consulting company, pause there: TN does not authorize self-directed American business activity. The employer must also be able to explain why the assignment is temporary, even where the role is expected to continue.