The exemption code depends on the type of entity that the W-9 is for, and the type of payments that the entity received. The codes are explained in the instructions on the W-9 form, beginning at the bottom of page 3.
Note that if you are filling out the W-9 as an individual or sole proprietor you are not exempt from backup withholding, so you do not have an exemption code. Leave it blank. If you are filling out the W-9 as a corporation, the exemption code would be 5 unless the corporation received a type of payment that is not exempt, as explained in the instructions. For other situations see the instructions on pages 3 and 4 of the form.
