I was reading a blog about Engineer when I cam ea cross a description of the roles each sPvP team would have (http://flame-throwing.com/):
The first three months (or more) in sPvP will be about exploration of tactics, team makeup and builds. But during the Beta events there have been a few common team makeups and roles.
Duelist A player that excels at 1v1 combat. This role it used to solo take an objective. Usually the closest or farthest objective depending on your tactics.
Roamer This player is used to support the other roles. Quick movement is a must as this role must listen to teammates to support them quickly and turn the tide of a fight.
Point holder Traditionally this role was a player that could take a beating without dying. The goal was to stay alive solo at an objective until support could arrive, like the roamer or the team. Guild Wars 2 makes this role difficult as surviving a 2v1 is more difficult than in other games.
Team Fight This role is designed to work in synergy with 1, 2, or even 3 other players to form a team (within your team). This team is usually used to capture and hold the most contested objective such as the middle objective on most maps."