HUD stands for Heads-Up Display, and refers to any atoms that appear on the screen but don’t move when the player moves. These are also called screen objects. Any movable atom can be added to the HUD by setting its screen_loc var, and adding it to client.screen for each user who is supposed to see it. This can be used to display a character’s vital stats, scores, etc.
If you want to have something like a health meter or name attached to a moving atom, use overlays or /image objects instead. An /image object is similar to a screen object in that it can be shown to only certain players instead of being shown to everyone.
The size of the screen depends on client.view (or world.view), world.map_format, and world.icon_size. In a normal topdown map format, client.view is the same as the screen size; in other map formats the screen might be a different size.
The screen_loc var can be set to a value like "1,1" (the southwest tile of the screen), "4,NORTH" (fourth tile from the west, along the north side of the screen), "SOUTHEAST", and so on. You can also include pixel offsets, percentages, and specify two corners to tile an icon repeatedly from one end to the other. See screen_loc for more details.
screen_loc can also be used to stretch the bounds of the HUD. A value of "0,0" will cause the atom to appear to the southwest of the southwest-most tile on the visible map, outside of the regular map bounds. Using HUDs in this way, you can provide a nice decorative “frame” for your map.
More complex
You can use HUDs in other map controls as well, by preceding screen_loc with the name of the map you will use followed by a colon. For instance, screen_loc="map2:1,1" will show an icon in the southwest corner of the map2 control. The actual size of a secondary HUD is based on how far out the icons in it extend in any direction. If you have one icon at "map2:1,1" and another at "map2:4,3", then that HUD will be four tiles wide and three high.