With deep links properties from referenced properties can be accessed for rendering.
Deep links make transitive selection of property values possible.
Assume that a property A in your projectdoc document refers to another document. To render the name of the referenced document you can use the following syntax in the Select parameter of - for instance - the Display Table Macro.
This navigation from document to property may be in any depth.
A -> Parent -> Type -> Name
In this example A, Parent, and Type are properties in projectdoc documents that point to other projectdoc documents.
Resources
More information on this feature.