There is a clear format which the employment letter should follow. It should mention duration of work, job title, main job responsibilities, hours worked, full time or part time. The visa officer will check if your job title and major responsibilities match or not. Further that should match with your claimed NOC, and job functions should match 70 percent or thereabout,
Also, it should be printed on company letterhead, which should mention company address, email , has a company logo, telephone. And should be signed and issue s by your reporting senior or HR.