Anyone who has successfully completed the selection process through EPSO and was placed on the Reserve List is eligible for an EU job in one of the EU institutions.
Becoming an AD or AST level European official means a permanent employment contract. This is covered in the Staff Regulations which spells out the rights and obligations of EU civil servants.
See also:
EPSO | Reserve List | EU Institutions | Administrators (AD) | Assistants (AST) | EU Recruitment | Staff Regulations
Related links:
Selection and Recruitment | How to Get an EU Job From the Reserve List