--> authorities, roles and responsibilities
Job descriptions are a minor source of these. Most are defined within procedures, work instructions, organizational charts and related documents. Even your quality manual will define some "...authorities, roles and responsibilities..." In ISO 9001 this also applies to the 'communications' requirement. If you have a corrective action procedure, for example, the procedure will typically contain who is responsible for what and thus lines of communication as well.
Job descriptions are often only a minor part of the whole when it comes to defining "...authorities, roles and responsibilities..." within a company.