Personnel certification bodies assess the competency of people: electrical engineers, career advisers, welders, inspectors, etc. It starts with an analysis. What type of job does a person have? What are the associated tasks? What skills are required? A personal certification body then examines how the competencies can be tested, for example via exams, assessment or a work piece. In the event of a positive result the relevant person is issued with a certificate. The major difference between this and a diploma is the certification involves ongoing supervision. A certificate therefore has a specific period of validity.
The RvA accredits personnel certification bodies on the basis of the ISO/IEC 17024 standard.
You can use the search engine or consult the list of accredited bodies to find details of all personnel certification bodies accredited by the RvA. Policy document BR010 describes the various fields of work relating to certification.