Magnitude
Modifiers to the Magnitude of Buffs or Debuffs come from whoever applies it and are multiplicative with modifiers to the Effect the Buff or Debuff has on the target.
Magnitude is a measure of the strength of the effect applied by some Buffs and Debuffs. It is usually specified in the source of the effect, but some buffs and debuffs have default values which are used if not otherwise specified.
Some buffs and debuffs do not have magnitudes as they are simply on or off, and hence are also not affected by modifiers to magnitude (e.g. Hexproof or Executioner's Presence).
For example, the magnitude of a Chill usually depends on the damage of the hit which inflicts the chill, but an effect that causes chill without a damaging hit has a default magnitude of 30% unless otherwise specified. In either case, modifiers like "Increased debuff effect" or "Increased chill effect" would then further multiply that magnitude.