The services agreement between the engineering firm and the plant owner, read together with the offer letter, decides who employs and directs the engineer. The file should show one employer of record and one reporting line.
Show the contract chain before the officer asks for it
When an engineering-services firm places a Canadian engineer at a U.S. client's plant, the TN application still names the firm as the employer. The officer will want to understand why the person is working at a different address, so the file should include the master services agreement or a summary showing the firm's scope of work, the client's site, the duration and the firm's retained control over the engineer's tasks.
The offer letter should match: same project, same site, same supervisor. A mismatch between the two documents is the most common reason a placement is questioned. If the client will direct the engineer's daily work and the firm merely issues payroll, the arrangement looks like staffing rather than professional services, and the applicant should raise that with the firm before travelling.
Keep both documents, a current licence record and the transcript in the same folder.