The “rules” for the certification processes are contained in a number of ISO/IEC standards.
ISO/IEC 17021:2011 Conformity assessment – Requirements for bodies providing audit and certification of management systems (The standard has been revised and is in transition as ISO/IEC 17021-1:2015)
ISO/IEC 17024:2012 Conformity assessment – General requirements for bodies operating certification of persons
ISO/IEC 17065:2012 Conformity assessment – Requirements for bodies certifying products, processes and services