Licensing
In civil aviation, the licence is issued under the rules of a state. The usual pattern is a licence, a rating for a type of service, and an endorsement for a unit.
The rating is what distinguishes aerodrome, approach, and area work. The endorsement is what ties the controller to a particular unit. A rating on its own does not authorise every unit of that type. The exact names on the licence vary by country.
Medical fitness is part of keeping the licence valid. The standard is the one set by the state that issues the licence, or the one the employer cites in its vacancy notice. It is not the same document in every country, and it changes.
How training leads up to that endorsement is covered under Training. Where the rules for a campaign are published is covered under Recruitment.