First of all, the "Source" is the base Code of all 7 Servers(Like Loginserver,Worldserver and so on) plus the Gameclient. So if you want to change some "bigger" things, you will need the whole Source to compile your new Servers/Client(Neuz.exe).
Also you should download some Files where the Source is already included, so basically you only need to change some paths and small things to make it work.
The following Threads may help you alot(for further Questions too):
