Guarding (often referred to in English as "blocking," not to be confused with Parrying) is a basic defensive mechanic that allows the player to guard (or block) enemy attacks in order to prevent or reduce damage.
In M.U.G.E.N, there is a bug with the guard implementation that causes attacks that come out on frame 1 of the state to be unguardable. Placing the following snippet of code in State 0 fixes this bug:
[State 0, Block] type = ChangeState triggerall = Ctrl trigger1 = InGuardDist trigger1 = command = "holdback" trigger2 = enemy,numproj >0 trigger2 = command = "holdback" value = 120
If control is returned to the player at any point during State 52 (landing), the same snippet should be placed in State 52 with the same triggers that determine when control is returned to the player.
Just Defend is a defensive mechanic that occurs in some systems (such as Garou: Mark of the Wolves, CvS2 K-Groove, Samurai Shodown 7, and Mortal Kombat 11 Flawless Block) when an attack is guarded shortly or as soon as an attack makes contact. Typically this gives some frame advantage to the player guarding, allowing them to punish the attack, and sometimes causes them to gain power or life as a bonus. Pushback may also be negated, allowing for easier punishes.
Just Defend can be implemented either the traditional way through HitOverride (or in rare cases, ReversalDef), or by overriding the guard states. Overriding the guard states provides the most solid implementation and is the recommended way to handle it.