Manual-01 Section 5.7 explains the difference between incorporated and referenceable items for project referencing. However, let's provide a bit more context. 

Referencable items represent all the items that can be picked as a relation target (e.g in the tested items field) and are also source for calculating Risk controls and other global arithmetics that revolve around references. Usually the referenceable space should be defined as a superset of the incorporated items. 

Incorporated items are items within the referenced project that act as if they were part of its referencing project. 

E.g my parent project incorporates all test cases from a sub project, so they now show up on all the relevant pages within the parent project. The way how the data shows up on the Ketryx UI is also the way how the data is rendered in release documentation.