From Ars Magica
Casting mode is the foundation of spell casting, and is an attribute available to all players.
Casting mode can be changed by pressing the Casting Mode Change Key (Default 'c').
There are three casting modes: Diminished, Normal and Augmented. Some spells are unaffected by casting mode.
Diminished casting is a weaker version of the spell, with a lessened mana cost. The icon for this is a down arrow or a crescent moon.
Normal casting is a standard version of the spell. The icon for this is an arrow pointing towards you or a half moon.
Augmented casting is a much stronger version of the spell, with a generally higher mana cost. Several augmented spells also require a Reagent. By default, augmented casting is locked, but can be unlocked by using the Archmage Podium which can be found in the Archmage Tower. The icon for this mode is an up arrow or a full moon
Some spell descriptions have the casting mode shown in calculations. In these cases, diminished is 0, normal is 1 and augmented is 2.