Why it is a separate requirement
In hazardous and very hazardous work, safety training and vocational training are different things. The second shows the person has the vocational competence to do that job, and it is not met by training the workplace gives.
Who issues it
It is obtained through the relevant vocational training or qualification system. The employer sees the certificate before work starts and places it in the personnel file.
The common mistake
Planning to complete the certificate after work has started. Work before the certificate's date counts as employment without one; a certificate obtained later does not act retrospectively.
In Optifora
The competence side matches roles to certificates: a person without one cannot be assigned to that role, and a certificate nearing expiry warns in advance.