Here is an explaination from Cessna concerning the issuance of Service Bulletins and Service Letters
Q: What is the difference between a Service Bulletin and a Service Letter.
A: Service Bulletins are technical documents that provide parts, instructions and approval for incorporation of an installation, modification or repair to a specific Cessna airplane by model and unit number. They are used to announce special product inspection criteria and modifications, revised maintenance information, operational information which is intended to assist in improving aircraft safety, reliability, durability, or performance.
There are four compliance categories for service bulletins and are defined as follows:
A. MANDATORY. Cessna's compliance level for a service document that must be accomplished for continued airworthiness under 14 CFR 43.13.
B. RECOMMENDED. Cessna's compliance level for a service document that modifies or changes and airplane system that are intended to improve dispatch reliability which should be accomplished within the specified time requirements.
C. DISCRETIONARY. Cessna's compliance level for a service document that modifies or changes and airplane system that is intended to improve dispatch reliability which can be accomplished at the discretion of the airplane owner.
D. OPTIONAL. Cessna's compliance level for a service document that installs, upgrades components, systems, or the airframe at the customer's request. Optional service bulletins are accomplished at the customer's expense.
Service Letters provide Information important to the maintenance and operation of airplanes. While Service letters might provide instructions on how to accomplish a task they do not provide parts and approval for the work accomplished.</STRONG>