It is a little bit more complicated than that.
At first you have 3 version of writables and they save different things. In pure basic it is the following
1. Profile only : Only saves the C:\Users folder and all HKCU settings (which is in that folder as ntuser.dat 
)
2. UIA only: UIA stands for User Installed Application. It saves everything besides the c:\Users directory
3. UIA + Profile: As the name states, it saves everything.
That being said, the truth is a little bit more complicated. If you look at the writable in option 3, not everything you save locally is automatically saved in the writable. If you look at the writable, there's a snapvol.cfg file. In that file you can add exclusions of files that you do not want to safe. If you look at it you will mostly see folders for virusscanners, Appvolumes itself and other stuff you do not want to safe.
Looking at how it works. When you log in with a writable the svservice (Appvolumes filter driver) checks to see if it needs to register information that you are changing in the machine. If it is excluded (this can be a folder or a process) it will do nothing with that information. If it is included it will safe it to the writable volume which is simply a mount point of a VMDK file. When you log off the disk will be attached but info being saved is written there because it is realtime. The moment you reboot a desktop the Appvolumes agent will detach the writable from the machine for you. Do keep in mind that wriatbles can only be used with a non persistent machine and you need to refresh the machine after log off.