Main Weapon - [A] (Square)
The primary ranged weapon of each MS in the game. This armament almost always some sort of beam rifle (BR) or machinegun (MG), though there are a few exceptions (G Gundam series MS, Deathscythe Hell, Epyon, Altron, Turn-A, Susanoo, GP03, Efreet).
These armaments typically reload in one of 3 ways: "normal" (constantly regen 1 shot every X seconds), "use-up" (the entire ammo stock is reloaded after Xs after you have used every shot of ammo), or "cartridge" (entire ammo stock instantly reloaded by cartridge change after you have used up every shot).
Melee Attack - [B] (Triangle)
The melee attack button of almost every MS in the game. There are though a few MS that have another ranged mode of attack instead of melee (Heavyarms, Cherudim, Raphael, Hildorf, Extreme Ef).
The type of melee attack performed can be altered using the directional stick.
Boost - [C] (X)
MS are maneuvered by using boost, which is limited by the boost gauge. When the boost gauge is empty you will be "overheat" and forced to land to allow the boost gauge to regen.
Pressing X once will cause the MS to rise. Pressing X twice will cause the MS to boost dash (BD) in the direction of the control stick. The various cost levels of MS have differing numbers of BDs before they overheat. Some MS have a mobile armor (MA) mode in which pressing a direction twice while holding X will transform them into a high mobility mode, consuming boost gauge at a constant rate.
There are a number of movement techniques employed in the game using boost that are discussed in the Boost/Movement page.
Sub-Weapon - [AB]
The secondary ranged weapon of an MS behind their main (A).
This slot is often where common armaments such as bazooka (BZ) and funnels are assigned, though certainly not always and not exclusively.
Special Shot - [AC]
Known for a long time in the Gundam VS series as "special shot" [AC] is often a MS-unique ranged attack, though, again, it's not always a ranged attack despite its terminology (in early VS titles this was exclusively a special ranged attack, which is where its name derives from).
This the button input to which Assist MS armaments are often assigned.
Special Melee - [BC]
Similar to [AC] this button input has historically been used to perform MS-specific melee attacks, though now in EXVS it is not a given that the attack will be melee at all, and in truth [BC] is maybe the most diverse button with regard to armament assignments.
Target Change - [E] (O)
Self-explanatory, how you switch which enemy is your current target.