From Halopedia, the Halo wiki
Typically, the Covenant fire their plasma in bolts, which is accomplished by forming a magnetic bubble around the superheated ionized gas - once fired, the magnetic bubble holding the plasma is guided towards its destination. Plasma fired in a narrow, concentrated beam is accomplished by modifying the magnetic field so that the superheated plasma is guided continuously in a straight line towards its destination. The focus cannon is usually reserved for assault platforms like the Sumda'te-pattern Scarab, Deutoros-pattern Scarab, and Shua'ee-pattern Locust. A focus cannon can be used to take down large scale objects, that either may be in the way of infantry objectives, or to excavate an area. If faced with a large military presence assault platforms can use their focus cannon to cut through armor and infantry with ease.
Locusts use their Charged Plasma Cannon to deal with structures that may be too small for a larger assault platform to destroy. Locusts may be able to perform hunter-killer roles, finding infantry and eliminating them. They can also fulfill an anti-armor role, capable of destroying heavy armor if needed.
Scarabs use their Ultra-heavy focus cannon to destroy large structures that may be blocking infantry objectives, or if an area must be effectively destroyed. Their main weapon can be used against vehicles if they are within the Scarab's line of sight. They cannot, however, be used in hunter-killer situations against single, or small groups of infantry.
Covenant capital ships also have a weapon similar to the focus cannon, commonly referred to as the energy projector. It is commonly fired from the belly of the ship right where the ship's central gravity lift is deployed when unloading troops and supplies.
List of appearances