GNU Emacs Manual
M.7.10 Customizing VC
vc-handled-backends determines which version
control systems VC should handle. The default value is
SCCS), so it contains all three version systems that are currently
supported. If you want VC to ignore one or more of these systems,
exclude its name from the list.
The order of systems in the list is significant: when you visit a file
registered in more than one system (see section M.7.7.2 Local Version Control),
VC uses the system that comes first in
default. The order is also significant when you register a file for
the first time, see section M.7.5.1 Registering a File for Version Control for details.