The profession, the duties in systems-analysis terms, the qualification relied on, the salary, the duration and the supervising U.S. entity. Anything the officer would have to guess should be written down.
Write the letter for an officer, not for HR
Hypothetical example: a U.S. insurer's HR team sends a Quispamsis applicant a standard offer that describes her as a senior member of the technology group and refers to an attached job family description. The attachment is a generic corporate grade description.
The TN letter needs to name the Computer Systems Analyst profession, describe the platforms she will analyse and the deliverables she will produce, and state that she meets the listing through her diploma plus three years of experience. It should confirm the temporary nature of the assignment, the salary and that the U.S. entity supervises her.
If foreign-language records support the experience, certified English translations must accompany them for a USCIS filing. Asking the employer to sign a letter drafted for the immigration purpose, rather than adapting the corporate offer, usually saves a second trip to the border.