Final answer:
An agency guideline is considered an opinion letter issued by an authoritative resource, providing guidance and interpretation of regulations or statutes by a government agency.
Step-by-step explanation:
An agency guideline is considered an opinion letter issued by an authoritative resource. An opinion letter is a written document that provides guidance and interpretation of regulations or statutes by a government agency. It is not legally binding but offers insight into how the agency views and interprets the law.