www.easaonline.com presents the different categories available within the EASA Part 66 licence
On the basis of the Maintenance Engineers License, together with satisfactory demonstration of competency and knowledge of organizational procedures the personal authorization document is issued by an approved Part-145 organization.
Note - Whilst Category A is not maybe the best choice to sit for your licence, clearly as it is incorporated within the B1 licence it can be of significant benefit to allow the B1 Engineer to be task trained on multiple aircraft.
A Category “A” License holder may only certify his (or her) own work and cannot certify the work of other individuals.
The Category “A” based approval is a task based approval which is restricted to the mechanical systems, although simple and limited avionic tasks as for example which may be found on a daily inspection or a weekly check, can be included with the approval of the Competent Authority.
(b) Categories A and B1 are subdivided into subcategories relative to combinations of Aeroplanes, helicopters, turbine and piston engines.