(a) The commissioner has the power to issue revenue and letter rulings, at the commissioner's discretion.
(b) Revenue rulings shall be statements regarding the substantive application of law and statements of procedure that affect the rights and duties of taxpayers and other members of the public. Revenue rulings shall be advisory in nature and shall not be binding upon the department.
(c) (1) Letter rulings shall interpret and apply the tax law to a specific set of existing facts furnished by a particular taxpayer. These rulings shall be binding upon the department and are applicable only to the individual taxpayer being addressed.
(2) Letter rulings can be revoked or modified by the commissioner at any time. Such revocation or modification shall be effective retroactively, unless the following conditions are met, in which case the revocation shall be prospective only: