Many game objects are placed in the world in one of two ways, absolute and relative. An object placed absolutely is in a fixed location and generally doesn't move unless scripted to do so and will stay there regardless of the player. An object placed relative is always a matter of relative to what. Often relative objects are relative to the player, though some may be relative to NPC's or other objects in motion.
The HUD is always an interesting question. Depending on your method and argument you can say the hud is absolute though it's generally relative, in cases where no hud really exists such as dead space it's not so clear. The health meter in that case is absolute on the player but the whole player is relative to the objectives which are absolute, meanwhile the inventory hologram is relative to the player. Sometimes the HUD is absolute on the camera other times it's relative to the camera view. Neither makes a big difference in relation to this but with other objects it can make a world of a difference. Adjusting to player skill is always a difficult process and it's important when designing these adjustments to understand relative and absolute positioning and variables.
Often when rpg's scale their enemies it's simply relative to player stat variables to equally adjust the enemy stats. However some games simply choose to have absolute values for an enemy which may make it impossible to defeat if the player hasn't prepared accordingly or extremely easy if the player was supposed to have encountered the enemy a long time ago.
This really isn't going anywhere, just good tip to know if you're making a game to keep it in mind, or it'll crush you. and I probably explained part of it wrong, but I'm also still recovering from being drugged, still a little loopy so.... yeah.