ci1=cr1 is a heads-up display (HUD) for user s1 of machine/system s2 based on principle ci3. It is shown/visible in front of cr2, as seen from the user's viewpoint.
A very general sense of “heads-up display” is used here; it is not limited to one single application. For example, it could be used to refer to both the heads-up display of modern aircraft as well as the heads-up display in computing and video games. Cf. crane, sazycimde.