Disclosure and Barring Service.
Employers may ask for your DBS (previously CRB) check before you start working for them to ensure you have no criminal records held. Most ask for the standard check (£26) but for jobs where you will be working closely with e.g. children, they may ask for an enhanced check (£44). It simply provides more information on someone that the employee may not tell the employer.
It can be relevant for some photography jobs is the agent requires one or you are working with sensitive clients.