It should identify the listed discipline, describe the theoretical knowledge and the practical problems the position addresses, and name the professional whose work the position directly supports along with that person's qualifications. A generic list of plant duties does not do this, however accurate it is as a description of the job.
Which document decides it, and who has to write it
The single most useful step is to ask the U.S. employer who will draft the letter and whether that person has ever written one for this profession before. A human-resources template will describe shifts, reporting lines and responsibilities in operational language, because that is what the template is for. What is needed instead is a description tying the position to one of the listed scientific disciplines and to a named professional in that discipline. Supporting material generally includes transcripts and training certificates, records of any internal method or procedure the applicant developed, and evidence of the equipment and analyses involved. Keep the original of everything and travel with copies rather than sending originals ahead. None of this guarantees an outcome, and a licensed U.S. immigration attorney should review the package before it is presented anywhere.
Before documents are gathered, ask the employer to identify the proposed supervisor, the worksite, the assignment dates, and the professional discipline that governs the position. That produces a draft letter capable of being checked against the credential evidence instead of a general recruitment note. A Canadian applicant may request TN treatment on arrival for qualifying employment, while an employer can pursue an I-129 petition; TN cannot be used to operate one's own U.S. business. Hypothetical example: A hydrologic-modeling specialist has certificates from three software vendors but no degree named in the offer. The useful sequence is to match each certificate to the claimed entry, then revise the duties if the records do not support that entry.