Mana Slots: Magic wielding Servants can cast only a limited he or she can cast only a limited number of magics before resting. Granted a limited amount of Prana that they must divide up between fueling their existence and their magic, this is manifested in Mana Slots, which represent how much mana a Servant possesses.
When a Servant uses magic, they expend a slot of that magic's level or higher, effectively "filling" a slot with the magic. Lower level magic can be fueled by higher level mana slots, effectively empowering the magic. A 1st-level magic fits into a slot of any size, but a 9th-level magic fits only in a 9th-level slot.
Finishing a Long Rest (8 hours) restores any expended magic slots.
Marie currently possesses on her, two level 1 slots. They may be used to cast both level 1 or 0 magic, the latter in an augmented form.
Level 0 magic does not require spell slots, and is unlimited in its' re-usability.