A heads-up display, or HUD, is a floating canvas in your field of view that overlays the gameplay scene. In VR vernacular, there are two variations of HUDs. I'll call these variations the visor HUD and the windshield HUD. This section looks at the first one.
In visor HUD, the UI canvas is attached to the camera. It doesn't appear to respond to your head movement. When you move your head, it appears to be stuck to your face. Let's look at a nicer way of visualizing it. Suppose you're wearing a helmet with a visor, and the UI appears projected onto the surface of that visor. There may be contexts where this is OK in virtual reality, but it is likely to break the sense of immersion. So, it should generally only be used either when the visor is a part of the gameplay, or if the intent is to take you out of the scene, such as the utility menus for the...