The position and its professional character, the duties, the named profession it corresponds to, the period sought, the terms of employment, and a signature from someone able to bind the employer.
A letter that answers questions instead of raising them
Hypothetical example: The strongest letters read as though written by someone who understood what would be asked. They identify the profession by name, describe duties that plainly belong to it, and explain any element that might look like something else — sales activity, supervisory work, or time spent on general operations. They state a period that matches the real engagement rather than a maximum, and they set out compensation and terms.
They are signed by an officer or manager with authority. Weak letters are usually recycled internal job postings: full of competencies, vague about the actual work, silent on duration. A Cavan Monaghan applicant should read the draft letter as an unfamiliar reader would and ask what question each paragraph leaves open.
Every one of those becomes a question at the counter, and the letter is the only place to answer them in advance.