In FPS games, traditionally there are two separate areas you can see. The first one is called the viewport; it's how you view the actual game world. In an FPS, it's the view from your character's head where you see other players, bullets, terrain, etc. If you use "cl_drawhud 0" in console, it leaves only the viewport. The second area is the GUI, normally shorted to just UI for video games. The UI refers to everything that is overlayed over the viewport and doesn't actually exist within the world. Now the complicated part is differentiating between the UI and the HUD. The HUD refers to the part of the UI that is always immediately available in most situations and exists solely to provide information to the player. Health/stamina/ammo counters, weapon selection, minimap/radar. These things all just provide the player information and can't be interacted with directly. I can change what it says indirectly by changing the state of the game; for example, I can fire a bullet and my ammo counter goes down, but I can't directly interact with the HUD to change the state of the game. I do not know of a particular term for parts of the UI that aren't part of the HUD, but almost all of the time it includes menus and parts that can be interacted with. For Empires, this is pretty much every menu that pops up if you press a hotkey. Research, vehicle menu, squad panel, team selection, class selection. All of these aren't parts of the HUD, but they are parts of the UI. As such, a UI redo implies a HUD redo, but a HUD redo doesn't imply a GUI redo. Nominally, this only applies to certain genres. RTS games aren't considered to have a HUD, only a UI, because you directly interact with it and it's always visible. (*For the record, I meant to post this like 30 hours ago but stuff happened and I don't want to delete it so I'm posting it anyway.)
