GUIDs in Trados

I have come across the following GUIDs used in a Trados project.
- Reference ProjectGuid
- ParentProjectFileGuid
- ParentTaskFileGuid

In all the project I have been looking at (including projects derived from one another) there were all zeros.
I tried to look online but could not find any reference to these.

Could you please explain
1. what they are for and
2. under which scenarios they would get a different value than zero?
Thank you very much for your help!