Double_r
As redtail says, Service Bulletins are issued by manufacturers to provide modification or inspection instructions. If the manufacturer thinks they are very important they may issue the SB as an Alert Service Bulletin. Neither type of SB is mandatory. The FAA issue Airworthines Directives that may either make a particular Service Bulletin mandatory or may impose actions of their own, such as requiring a revision to the Airplane Flight Manual.
In your question you mention that your paper is for an Aviation Safety class. Perhaps you are really referring to the FAA's Special Airworthiness Information Bulletins?
SAIBs are not mandatory but provide safety information that is deemed important.
Visit:
http://av-info.faa.gov/ad/SAIB%20PAGE.htm
to find out more about Special Aviation Safety Bulletins while you can look at a few Airworthiness Directives to get the general feel of them at:
http://av-info.faa.gov/ad/Defaultad.htm
Note that FAA ADs may also be mandatory on non-US registered aircraft too. For example, UK regulations make any AD issued in the State of Manufacture automatically applicable to UK registered aircraft.
I hope you get an "A"
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Through difficulties to the cinema