Many built-in properties, such as Object.parent, reference scene data. In the UI, they are represented with pretty menu widgets and in certain cases also with a picker tool:
Is it possible to create these 'reference properties' from Python? For example, to create a custom property Object.step_parent.
To some extent, it can be emulated by a props.StringProperty -- but the real reference does not break when objects are renamed, and provides a better user experience in general.
