The "trunk" series represents the primary line of development rather than a stable release branch. This is sometimes also called MAIN or HEAD.
This is the series for all 1.2.x releases.
This is the 1.1.x series of UFL, the place for new features after FEniCS 1.0. Until a release date is planned, development intended for 1.1 will happen in trunk.
This series corresponds to dolfin and ffc 1.0.x.
This series collects all the pre-1.0.x releases.