ARFoundation: What causes a subsystem to be null?

Hi,

I'm looking for some clarification on how subsystems work in ARFoundation. I can't find anything definitive in the doc.

1.What causes a subsystem to be null?
2. Is it safe to check if a subsystem is null just once in a Start() function? In some of the examples I see it's getting checked in an update loop. Do they take time to "turn on"? Can they randomly be null on a frame?
3. Is it safe to use subsystem != null to check feature support? In the ARCollaboration sample scene I found this code which doesn't work at all:

Code (CSharp):

1. void Update()
2.     {
3.         var subsystem = m_HumanBodyManager.subsystem;
4.         if (subsystem == null)
5.         {
6.             if (m_ImageInfo != null)
7.             {
8.                 m_ImageInfo.text = "Human Segmentation not supported.";
9.             }
10.             return;
11.         }
12. }

The system isn't null on an old device that definitely doesn't support segmentation. Instead when I check for

Code (CSharp):

1. subsystem.SubsystemDescriptor.supportsHumanDepthImage

I get the right result.

Some subsystems also have a dedicated support check:

Code (CSharp):

1. ARKitSessionSubsystem.supportsCollaboration

What's the correct way to check if a feature is supported?

Thanks!