What are service letters and service bulletins?
In the United States and presumably in other countries, there is no Government oversight on how a manufacture communicates to aircraft owner/operators except that they must issue some form of mandatory compliance "Service Bulletin" for Part 135/121 commercial operators. There is no industry guideline for what a document is called, or the information it conveys.

Manufactures often use an assortment of documents and the type of information in each document is left up to the company. For example, Continental Motors uses almost every kind of document I have ever seen...


"Service Bulletin" i.e. SB "used by most manufactures..."
"Critical Service Bulletin" i.e CSB 01-1 "Fuel Pump Inspection..."
"Service Bulletin Mandatory" i.e M64-18 "Field Conversion to
    Turbocharging..." 
"Service Information" i.e. SIL99-2A "Current Listing of Sealents..."
"Service Information Directive" i.e. SID97-4C "Cylinder Bore And Piston Fit..."
"Service Letter" i.e 556c "Shouldered magneto drive shaft bushing..."


There are others I don't have copies of such as information sent to marketing, promotions, or financial departments. Additionally, the FAA will require specific forms to be used as part of the approval for Part 135, 119, and 121 operating rules. For example some type of "trace-ability" paperwork is required to be attached to all materials and parts. A "trace-ability" document provides manufacture, model, mil-spec, fire resistance, etc.

It is interesting to note that despite the universal use of the Red, Yellow, and Green parts tagging system, their is no FAR regulation requiring it's use.