SDK: Software Developer Kit.
These are packages for the SDK cross-compilation setup on your PC, i.e. where you develop stuff that you can later install on the N900. As there are armel versions (which run in an emulator on the PC) of packages some of these will work on the actual N900 itself too. But you have to be careful.

So, it's not yet-another-repository for the N900: We're still at extras, extras-testing, extras-devel.