Documentation For ProjectGuid? #3923

The ProjectGuid tag is pretty important in the MSBuild Ecosystem and as the name suggests should not be duplicated.

However I am unable to find documentation that describes just exactly how this is used.

At very least I know it is used when you create a ProjectReference Tag (and is documented here https://docs.microsoft.com/en-us/visualstudio/msbuild/common-msbuild-project-items?view=vs-2017#projectreference ) and when you create a Solution File in Visual Studio (the Project Guid is extracted and then kept in the SLN file) that is not as well documented but you see some inklings here: https://docs.microsoft.com/en-us/dotnet/api/microsoft.visualstudio.workspace.extensions.msbuild.projectinsolution.projectguid?view=visualstudiosdk-2017 ).

Can someone point me in the right direction? If its not documented, who should I forward a request to?