Heads-up display

Summary
The Heads Up Display, or HUD for short, is the main source for information in-game on Halo: Combat Evolved and Halo 2.

Usually found on the inside of helmets of various types of body armor, HUD's are the standard, interactive interfaces for the user of a piece of armor. The information readouts they provide are vital to the survival of their user. They are featured in MJOLNIR armor, ODST Body Suits, and even Elite armor.

Notes about HUD
about grenade counters: both items 1 and two are replaced by the ammo counter of the left hand weapon when dual wielding in Halo 2. There is also only one interchangeable grenade counter in Halo 1

About alternate weapon icon: there isn't one in Halo 1

About motion tracker: sometimes disabled in multiplayer. crouch walking doesn't register on motion tracker.

About normal HUD: the HUD for the Arbiter/Covenant Elite is purple instead of the Spartan blue in Halo 2

About Energy Shields/Health: In Halo 1 the energy shields were at a different position on the HUD and had a non-rechargeable Health counter below it. The health counter was scrapped in Halo 2 in favor of a rechargeable, invisible health system that recharged when the shields recharged.

External Reference
HUD