Bradley It is a common convention when drawing circuit diagrams not to show the GND (-ve) rails, simply to simplify the diagram and avoid clutter. It is self evident that all powered components must have a current return path. So don’t worry about the lack of B- rails!
The diagram you attached, like most Dufour wiring diagrams which I have seen contains several errors, as a consequence understanding / fault finding can be tricky.
The diagram is incomplete, if you have / can post the rest, including the key then I’ll have another look. At the moment i suspect “C-” is another ground rail BUT either switched / controlled in some way or from another battery / power source.
The membership association for owners of Dufour Yachts