This is the repository from where the new changesets are retrieved. It can be one of the following kinds:

 1. BzrngRepository
 1. CvsRepository
 1. CvspsRepository
 1. DarcsRepository
 1. GitRepository
 1. MercurialRepository
 1. MonotoneRepository
 1. SubversionRepository
 1. BazRepository
 1. TlaRepository
 1. [wiki:P4Repository P4Repository]

If you're trying to convert a BitKeeper repository, you can first request it to be converted to CVS by BitMover themselves, using http://kernel.bkbits.net/cgi-bin/cvsexport.cgi, then import it using cvs or cvsps sources.  CVSps should be able to extract bkcvs-specific information from such repositories, but tailor still has to support this.