Competence is task based. You have to ensure that all your employees (and contractors, agency staff, temporary workers, etc.) have the qualifications and experience to carry out their tasks safely.
In certain circumstances, some of which are outlined in the table on the previous page, competence can be demonstrated by possession of a certificate or licence, but you must still supervise employees effectively.
It is recommended that you have a system in place to identify and regularly monitor (through an annual appraisal or similar) the competence of employees and use this to identify training needs and how to fulfil them.