First off manufacturers do not make regulatory rules regarding the (re)fitting of parts or procedures. They are recommendations only and are submitted to the regulatory authorities for their juristiction.
So why does AirBus label some of them as "mandatory" and others not.
Could you please explain ?
If communication from OEM is coming as a result of an AD (Airworthiness Directive) then it is mandatory. If it is a SB (Service Bulletin) then it is not mandatory.
Recommendations are always optional. Only ADs are mandatory, with the appropriate time frame for application.
Regards,