Average salaries for all professions are available through the Bureau Labor Statistics, or BLS. The bureau maintains a fair amount of these averages and means for comparison. The IRS uses your occupation entry as a tool for determining if you are misrepresenting your income. In addition, the entry adds a bit of context to the deductions and credits that you claim on your return. For instance, a business owner may have a great deal of losses during the year, so the losses listed on her schedule C make sense. But a store clerk wouldn't have business losses and thus, such an entry would raise a red flag with the IRS.
In the instructions for form 1040, it states that taxpayers are to date their returns and list their occupation(s). The fact that the IRS chose to use the plural versus the singular tense implies that taxpayers can write more than one occupation in the designated area. The IRS advises tax preparers that they can enter only the code of the occupation that produces the greatest amount of income. If you file a joint return, you have to list the occupation of both yourself and your spouse.
If you cannot properly define your occupation, enter a description instead of the name of the title of your occupation. You will also find that several headings cover a variety of employment types.