Magicians cast spells by drawing energy from astral space and forming it into a pattern. The pattern of each spell is unique and defines what effects and limitations it has. A magician requires the appropriate Thread Weaving talent in order to weave threads to create a pattern, and the Spellcasting talent allows the pattern to manifest and cause an effect.
Spells are traditionally placed in a Spell Matrix to shield the magician from corrupt energy drawn from astral space when forming the spell. In contrast to other role-playing systems, there is no limit as to how many times per day a spell may be cast. Thus the number of spells a magician has access to at any one time is (typically) limited to the number of Spell Matrices that they possess.
Once a spell is placed in a Matrix, it remains there until the magician attunes the Matrix to another spell (or is dislodged in some way). When not in a Matrix, a magician still has access to the spell and may cast it foregoing the benefits that a Spell Matrix provides.
Methods of Spellcasting that do not employ a Spell Matrix include casting using Raw Magic and casting from a Grimoire. Spellcasting using these methods is a more hazardous proposition than casting through a Matrix (dependant upon location and circumstances), as magicians risk taking damage from corrupt astral energies or being Horror-marked.