The Absolute Defense Field, or AD-Field, is a barrier that BST-Frames can generate for defense or, in special situations, movement or even offense.
The AD-Field is a barrier that is usually invisible until it comes into contact with other objects. They are primarily used as a powerful defensive shield that is impenetrable by normal means. While the mechanics of the AD-Field are not entirely understood, it is known that there is no singular power source for an AD-Field within a BST-Frame. Instead, the AD-Field is generated naturally throughout the entire frame, as BST-Frames were constructed from the corpses of Prophets which are capable of naturally producing the AD-Field, the unknown material they are made of acting as a conduit for the field. During The Beast, the BST-Frame's armor segments separate to reveal a glowing frame underneath, which further increases the power of the AD-Field and presents more uses for the field, such as for movement and even for offense. The AD-Field is integrated into the BST-Frame by the very nature of its construction, with the system being integral to the frame's operation.
The only known thing that can effectively defeat an AD-Field is the AD-Field of another BST-Frame, which, when resonating at the same frequency as an opposing AD-Field, will cancel each other out, nullifying both AD-Field for a short time and allowing attacks to slip through. Aside from this, physical weapons in general have the greatest effect on AD-Fields, though it requires an incredible amount of force concentrated on a small area of the field to breach it. Energy weapons are able to achieve this, as well, though it is more difficult as the energy is more easily dispersed than a physical object. It is also possible to whittle down the power of an AD-Field over time through continuous attacks, although it takes a considerable amount of effort and time to do so. During The Beast, however, all attempts to break the barrier become impossible, aside from another AD-Field.
However, the mechanics of an Executioner Type BST-Frame's AD-Field operate in a different way. The Executioners, being designed specifically to destroy other BST-Frames, do not use the normal method of nullifying an AD-Field by matching frequency, though they are capable of doing so all the same. Instead, an Executioner is more suited to shatter an enemy's AD-Field entirely through sheer force, and is able to utilize its own AD-Field in more esoteric ways in order to bypass the need to give up its own defenses in order to penetrate those of its target. Exectioners are capable of using their AD-Fields in a much more aggressive manner than normal BST-Frames, being able to concentrate the power of their AD-Fields to single points in order to damage another AD-Field without compromising its own integrity.
An AD Burst refers to an ability of Executioner Type BST-Frames to release their AD-Field in a powerful burst around them, most of the time completely destroying whatever it hits. Though the burst is powerful enough to destroy most unprotected targets instantly, the ability is mainly used to shatter the AD-Fields of other BST-Frames, primarily while engaged with multiple BST-Frames at once. Though the ability is extremely powerful and effective at eliminating other AD-Fields, it comes at the cost of the Executioner's AD-Field also being broken and therefore unusable for a limited time, leaving itself vulnerable in order to create an opportunity to destroy its targets.
An AD Cannon refers to the use of an AD Field as a powerful offensive energy cannon, more often than not capable of completely annihilating anything caught in its blast. The power of the weapon is near indescribable in how much damage it is capable of causing. The AD Cannon, however, seems to be an extremely rare armament on BST-Frames, only being seen on two units so far, being BST-13 Fenrir and BST-16 Muramasa. The AD Cannon of BST-13 requires the frame to be locked onto the ground and has a long charge time in order to accumulate the power necessary to fire the cannon. When fired, the unit's AD-Field is completely drained for a time, being longer than if the shield was simply broken normally. However, BST-16 features a smaller version of this weapon as its signature weapon, used at point-blank range to annihilate an enemy captured in its limbs. As the weapon on the Muramasa is less powerful than that of the Fenrir and is designed to be used at short range, the effect on its AD-Field is much less detrimental, though still weakens the field upon repeated use. The effects caused by an AD Cannon is similar to that of an AD Burst, though concentrated into an extremely powerful beam of energy, thereby causing much more damage over a smaller space.