This answer is not 100% correct, but only one way to do it. Since most people have more rooms than streams that they have (8-16 vs 2-6) it can be very annoying for people to see 8+ airplays and or confusing when they can't play that many at a time. Personally my company sets up 1-3 airplays or one per household member who needs it or how many possible streams they actually have. We also always disable phone volume for the same consistancy that control4 app/remote/touch panel is your only volume control.
So they only have two options start airplay on phone then jump to control4 app to turn on the zone they want airplay on or vice versa since they use the control4 app for volume for everything else. I find it hard to explain if we make an airplay per room that this one source you can use your phone's volume but if you add more rooms in the control4 app it will be a master volume. Time consuming on the programming due to volume level setting when source selected and or if they add rooms to the mix it gets complex in raising there volumes. Then there is the issue of setting this volume high for this one source so they use their phone volume and they have the volume up really high on their phone and it blasts.
Yes you could make a group airplay to eliviate the complex programming I mentioned, but they you see even more airplays when looking for the one you are looking for. In my opinion, this is why its easier to just have a default name such as C4 airplay, C4 Airplay 2, or Dad's C4 Airplay etc as the airplay name and know that you have to do the rest in control4 app.