The portfolio hierarchy is defined per workspace. It is dynamically set by a workspace administartor.
The PortfolioItem endpoint is a parent to any and all portfolio item hierarchy items. Let's examine an example.
In this example your workspace has the following hierarchy for portfolio items:
For such a workspace, Rally will dynamically generate 3 new endpoints called: "PortfolioItem/Theme" , "PortfolioItem/Initiative" and "PortfolioItem/Feature". All these endpoints will share a common parent endpoint called: "PortfolioItem".
What this essentially means is that the common fields between your portfolio hierarchy levels is defined on the parent - on the PortfolioItem endpoint, while specific functionality or behavior can be defined on any particular level.
Furthermore, the dynamically generated endpoints for the three levels will include a full REST interface for Create,Update,Read and Delete which you can utilize by http.
You can also read more here
, on the relationship of the portfolio hierarchy and user story hierarchies.