The French system is completely different from the Belgian system, the Belgian system is more "complicated" (not to change!). My explanation will complete that of elephant, point of view "small project leader".
A Green Certificate is a "subsidy" on the virtual market for green electricity produced.
The more “green” MWh is, the more corrections there are on the CV and the more value it increases. This can surely be subject to variations. It is a virtual market which is added to the real market! Thus the Walloon green kWh is paid twice: once actually and once virtually!
Thus, you can very well use energy internally and benefit from CVs (impossible in France which is stupid), just add a lead meter to your energy source before internal use. It's an extremely interesting asset but can be the source of abuse (and I know what I'm talking about ... because I saw a real one)
In France, you sign a buyout contract on REELLE electricity injected into the network for X years with EDF and it does not move. This only concerns the real market!
Here is a complete document on the method of calculation of green certificates but maybe this is the one you're talking about?
Be careful as much in France as in Belgium, obtaining certification is not easy. Except on the PV where these are facilitated systems.
Here I think I have not told too many stupid things.