The Matrix pack is a prototype Terran personal shield.
It was developed after the First Great War. Many terran defense companies tried to salvage some Protoss plasma shield technology in the hopes of fielding their own variant of personal shields (Stargate - Atlantis: Personal shield emitter). Through many trials and errors, a company from the Umojan Protectorate managed to reduce the size of the temporary ones used by Science vessels into a backpack shaped device capable of fitting most powered armor.
These Matrix packs were simplified to two settings: personal and dome. The personal setting is the stronger of the two as it is able to take the brunt of a Siege Tank shot before needing to recharge. On the dome setting, the packs widen the radius of the Defense matrix allowing more than one person to be covered (Halo 3: Bubble shield). This setting drains the pack's power supply fast and is best used for surviving a single powerful attack rather than sustained fire.
Due to expensive costs in manufacturing, Matrix packs were limited to lab testing and select military forces.