Individual components will work by themselves. This is one of the diagnostic features of DI2. A fully functional component should pass all of the test tests on the computer that's running Shimano's DI2 software on. In a set of circumstances you will need a junction A and Junction B. For the most part its just a matter of daisy chaining components together into a workable order. The DI2 system is a fairly rudimentary dumb computer system. The only component that locks you into a DI2 system are the derailleurs that determine whether your bike is a 10 speed or 11speed DI2 bike. Other than that everything is compatible for DI2 road except the first generation 7970 Dura Ace DI2. You can even put 9070 components on a 6770 bike once you have the front and rear derailleurs and the basic firmware wont know any different. You just wont get the 11th gear.