Mastery – like a stat.
Nominate two of your abilities, (Body, Finesse, Mind, Charm, or Sense) one as your “Reserves”, the second of these as your “Burn Out”. These are the energies your magic taps into, when you go beyond simple spellcasting. These are also the abilities you risk damage to, when your spells go awry, or overtax you.
Each spell you know is a skill.