It means, 'not applicable.' CIC uses the MAIN DUTIES to determine your NOC. If "employment requirements" state that you need a degree in accounting (for example), but you are working as a computer programmer, even though your degree is in (for example) history, it does not matter for CIC. If you are performing the the main duties listed, you will have your work experience classified as a computer programmer.
It's a little different if it is a "regulated occupation" like most health care providers. You DO need to be properly certified and licensed...