It has to establish the professional nature of the duties, the claimed profession, the period and terms of engagement, and who the employer is. It cannot establish citizenship or the applicant's credential.
What the letter can and cannot carry
A support letter is evidence about the job, not about the person, and files often fail because it was asked to do both. It should describe the actual work in operational language, identify the listed profession being claimed, state the intended duration and the arrangement under which the applicant is paid, and name the employing entity clearly.
Credential proof comes from the awarding institution or the licensing body, and citizenship comes from a citizenship document; a letter asserting that someone holds a degree proves nothing about the degree. Where an applicant will spend time at a third party's site, the letter should also make plain who directs and controls the work, because that arrangement affects whether the described employment is what it appears to be.