Do you need to allow the user to select a screen position? Would you like to provide some feedback as they move the cursor around the screen?
If so, the SpecifyScreenPosition function and the overlay graphics primitives are just what you need. The SpecifyScreenPosition function allows you to define a "motion callback function" that will be called each time a mouse movement is detected. If you have created any Windows forms applications, this would be very similar to the MouseMove event. The motion callback works with Overlay Graphics Primitives (OGP) to allow for some cool graphical effects that are very useful for user feedback. Think about using a selection rectangle or lasso in NX; there is visual feedback regarding the size and position of the selection method, but they are just temporary objects - you don't end up with four new lines in your part file each time you use a selection rectangle.