The job description on the tax return is apparently not very important. A word or two is sufficient - like manager, physician, route sales, clerical services or whatever is appropriate is all that is needed on that form. Include enough that the IRS can see the relationship to any significant expenses you have, like a lot of mileage. But if you have such significant expenses you would be filing Schedule C. We do not always include the business code and have had no issues with that from the IRS.If you have an EIN for your business you should include that so the IRS knows the business return has been filed.