If the SB is not supported by an AD
Service bulletins do not have to be enabled by AD. They do however have to be approved by the civil airworthiness authority of the aircraft's state of design.
In some countries, the term "Mandatory Service Bulletin" is used to define changes that are the result of Airworthiness Directives.
Service bulletins by their very nature contain information that supercedes the AMM. Why bother issuing supplementary documents that just parrot the AMM?
Not all service bulletins are mandatory therefore technicians should work with their engineering/maintenance departments to determine if the contents of any particular SB will be adopted.
And...when working with a service bulletin, follow it to the letter. Do not mix in AMM content without consulting the OEM.
Many SBs are produced as a result of an operator asking for reduced tolerances or greater flexibility. So don't be surprised to see SB tolerances deviate from the AMM.