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XR Session Subsystem
A "session" refers to an instance of AR. While the other AR subsystems provide specific pieces of functionality, like plane detection, the session controls the lifecycle of all AR-related subsystems. Thus, if you Stop (or fail to Create) an XRSessionSubsystem, the other AR subsystems may not work.
Start and Stop will start/resume and pause the session, respectively.
Determining Availability
On some platforms, AR capabilities are built into the device's operating system. However, on others, AR software may be installable on demand. The question "is AR available on this device" may require checking a remote server for software availability. Therefore, XRSessionSubsystem.GetAvailabilityAsync as a method that returns a Promise<SessionAvailability>. Promise is a CustomYieldInstruction, so it can be used in a coroutine.
Once availability is determined, the device might be unsupported, supported but only with an update or install, or supported and ready.
Installing Additional AR Software
If SessionAvailability is SessionAvailability.Supported but not SessionAvailability.Installed, you should call InstallAsync to install the AR software. This returns another type of Promise: Promise<SessionInstallationStatus>. If the installation is successful, then it is safe to Start the subsystem.