Deployable cover

Deployable cover is a new piece of Covenant Equipment in Halo 3. Its symbol in the HUD is the outline of a dome with a small "tick" mark inserted inside it.

This shield is similar to the Stationary Shield Generator of the same type seen in Halo: Combat Evolved and Halo 2, except it can now be deployed anywhere(probably replacing it). Additionally, the shield operates as a one-way shield - the user is able to fire through the shield from behind it with projectile weapons, but any bullets and other solid projectiles (except Carbine rounds and Needler) coming from the front will be deflected, however plasma fire and Spartan Laser's are stopped by the shield from either side. However, the Spartan Laser goes through around five deployable covers, assuming they are lined up close together. The user is given an enormous tactical advantage in exchange for being exposed from behind and to the left and right, whereas the similar Bubble Shield provides complete 360 degree cover, but cannot be fired into or out of.

The shield is deployed in the same manner as all other equipment in the game, and it will stay activated until it takes enough damage. The shield's health is represented by its color: it is normally blue, turns red/orange as it is damaged, and finally deactivates. If left alone, the shield will regenerate to full health. To deactivate the shield permanently, the base must be destroyed. This only works in campaign. In multiplayer, the deployable cover will self destruct after 30 Seconds. It is also possible to make a "bubble" from two of these shields.

Like the Portable Gravity Lift, the Deployable Cover, when activated, will be destroyed after a fall from a great height.

Glitches
One of the most popular glitches that is used with a deployable cover is the HEV crate glitch. This glitch essentially allows you to temporarily create a sort of HEV. It is used in conjunction with a gravity lift and an open crate. What you must do is find an open crate and place the gravity lift inside and then get the deployable cover and then throw it on top. The gravity lift will then lift up the deployable cover which in turns pushes on the crate and causes it to fly into the air. It may take a couple of tries to get this right and the only reasonable way you can pull this off is by arranging it on forge or having a partner. The crate will fall when the gravity lift runs out, but firing from in here gives a good battle field vantage point and the player is only truly vulnerable to explosions. When you're against a Scarab and it's about to explode, put down a deployable cover to protect you from the blast.

Trivia

 * Deployable cover is often used as an area to jump off of, as it is easier to use than the grenade jump for some players. One of this tactic's best uses is to get the Famine Skull in Halo 3.
 * Some people use it as wall to block doorways in order to slow down the enemy and regroup with their team.
 * It takes three melees to the shield itself to take it down.
 * There is a glitch that allows the player to become stuck, and have the Deployable Cover spawn through them.
 * Deployable covers were seen in Halo:CE and Halo 2 in Campaign sometimes used by Elites, but these were immovable and while they could be deactivated, they couldn't be destroyed.
 * The only map the Deployable Cover isn't available in Forge mode is Sandtrap.
 * In Campaign mode, the Deployable Cover is completely immune to the projectiles fired by Flood Ranged Forms making it very useful on the eighth level of Halo 3, Cortana.
 * The other main problem is that if you stand to close to a portable shield then you still take damage (although not as much) and can be severely damaged when the shield is destroyed.
 * The Deployable Cover seems almost immune to ballistics but can be depleted rapidly by energy weapons such as the plasma rifle and plasma pistol overshot.