On a form, N/A means you should not try to answer that question — it does not fit you. If a box says “name of husband or wife” and you are not married, you write N/A instead of inventing something or leaving a blank that looks like you forgot. People read it letter by letter, “N A,” or as “not applicable”: this item does not apply. It is almost only written. Though actually, everyday use is messier than the tidy dictionary line. People also write N/A when they mean “I haven't got this” or “not available” — no data, no product, no answer. Offices are not always careful about which one they intend, so you read it from the form around it. It does not mean “I don't know.”