In the WSAPI docs we see that Milestone has a parent called: "Slice". What is the Slice object?
<Please see attached file for image>
The Slice object inherits from WorkspaceDomainObject .
The Slice object is independent of Projects or Artifacts. The idea is to use it to specify objects that can be associated with multiple projects and multiple artifacts. Normally artifacts live in the context of a project. Yet, the Slice object can associate artifacts across numerous projects and hence provide an outlook across a few projects.
Two objects currently implement the Slice object. There are: Milestone and Investment.