project tree vs project hierarchy vs subprojects

Hi all,

The Version attribute of a project has a 'sharing' flag I do not understand.
What's the difference between the three of 'project tree' vs 'project hierarchy' vs 'subprojects'?
My Project has subprojects, so I do understand the subproject sharing but I cannot find any information about the 'project hierarchy' or 'project tree' definition.

Is there any link/doc about this?