[Virtools] Coordinate System & Orientation

I am not familiar with the coordinate system(s) in Virtools Dev and I need and welcome a help. I understand that there is a 'global coordinate system' ('World coordinate system') by means of it I can place (or translate, rotate, ...) 3D objects in the World. Besides, there is(?) a 'local coordinate system' of a 3D object, whose axes can be set with respect to a 'referential', e.g. with respect to --NULL-- ('World coordinate system') e.g. in authoring mode in the '3D object setup' -> 'Orientation World' (using Euler angles) or in run-time by the 'Set Orientation' BB by the top parameters 'Up', 'Dir' and 'Referential' set to '--NULL--' or another 3D Entity (3D Object). I do not understand what it means 'Local Up' and 'Local Dir'. I have supposed that these parameters set an orientation of the 3D object with respect to the 'local coordinate system' of the 3D object but it does not work this way. I carefully read manuals, online help and tried e.g. the 'Orientation and the Referential.cmo' but it did not help me. Can anybody explain me how the coordinate system works in Virtools Dev?