How can FishEye help with merging of branches in Subversion? — In merge management, the main advantages of FishEye come from its search functionality. If you record the revisions merged when you check in a merge result, you can find this information in FishEye easily for the next merge operation.
Why do I need to describe the branch and tag structure for Subversion repositories? — In Subversion, branches and tags are defined by convention, based on their path within a repository, and not directly defined by the repository. A few different layout alternatives are commonly used. It is also possible that you are using your own custom layout. As a result you need to describe to FishEye which paths in your repository are used as branches and tags.