How to attach an object to your screen?
By choosing the HUD_FULL_SCREEN bone, you can attach an object on your screen (in fact on your camera). You will be the only one to see it.
The object will hide your view, it can be used for example to create dark glasses to hide the world. The object will be stretched according to the resolution of your Planet window in Windows. Try resizing your Planet window to see the effect.
The HUD_HORIZONTAL and HUD_VERTICAL bones can be used to place clickable icons on your screen (place a script in the object to perform an action).
Choose HUD_HORIZONTAL to place an icon at the top or bottom edge of your screen, and choose HUD_VERTICAL to place an icon at the left or right edge of your screen. When you change the proportions of your Planet window, they are stretched in different ways.
Try to modify your Windows window to verify that the icon stays on the screen. With HUD_HORIZONTAL, if you place an icon at the bottom edge of the screen, it is advisable to set the bottom edge of your mesh to Z = 0, so that it does not slide off the screen when you change the height of your Planet window. In the same way for an icon at the top of the screen, set the highest point of the mesh to Z = 0. And the same for HUD_VERTICAL, align X = 0 on the mesh with the edge of the screen where the icon should be placed.
Note that not all users have the same screen resolution, so try resizing the window to make sure your object is visible at any screen size.