Source code for createIndex() ?

[d_stranz]

The third parameter is also defined as a void * pointer, which in reality means it can contain anything that will fit into the size of a pointer (usually the same size as an unsigned long). In Python, you could possibly use this to store the index into a list of objects, for example. The implementation of PySide might even coerce a Python object reference into a pointer "under the hood" so you may be able to store a Python object reference in it.

But as anda_skoa says, it is rare that you need to use this third parameter unless you are implementing a tree or a proxy model where it would be difficult to navigate the model otherwise.