What are these topic branches in the Ubuntu Kernel?
Normally all of the source code for the Ubuntu Kernels are stored on the master branch of the Ubuntu git repository for a release. However where there are significantly different kernels for different systems, for example it may be at an older mainline level, or may contain very invasive changes, those cannot be merged safely into the master branch. In this case the source code is maintained on a topic branch (see Kernel/Dev/TopicBranches). We use a split debian system to allow these to cleanly coexist in the same repository (see KernelTeam/AbstractedDebian)