Headcannons to my various universes.
Unicorn Magic Classification and Hierarchy
Volantus - Magic formed by the caster's will or desire, free form, and doesn't require magical formulae like Arcanum.
Arcanum - Most spells are in this type of magic. Arcanum uses magical formulae to function. Generally unicorns who are skilled in mathematics and the sciences are better with Arcanum spells than less educated unicorns.
Volantus/Chaos Magic - Magic that alters probabilities for a desired outcome. The effects happen over a time limit specified by the caster, it requires more power to have the desired outcome occur in less time. Unicorns with immense mana reserves, can generally get a desired outcome in about ten seconds, while weaker casters will take longer. Skill comes into play differently than most magics, as any unicorn is capable of using Chaos magic (although most don't have the mana reserves to cast such magic). Since chaos magic affects the world to get a desired outcome by skewing probabilities to cause unlikely events that chain until the caster gets the desired result, skill is required to prevent the magic from altering the world in ways the caster doesn't want. The most skilled Chaos Caster is Discord, with his nearly infinite mana reserves and unmatched skill with chaos magic he can get desired results with exactly as little (or much, if he wants to be surprised at the changes) environmental altering as he desires, in near instant timeframes.
Volantus/Charm Casting - Charm magic is magic that doesn't follow strict formulae, instead the caster impresses what they want the spell to accomplish into the magic as they cast.
Volantus/Charm Casting/Emotion Magic - Emotion Charm Casting uses emotions to help impress the caster's desire into the spell, while somewhat limited in terms of what can be done, based on the emotions used, any basic magic user can accomplish emotion spells. Emotion spells tend to require more power than Arcanum, so most unicorns can only do low power spells.
Volantus/Charm Casting/Emotion Magic/Positive - Emotion driven charm magic that is impressed upon with positive emotions or feelings. Love, friendship, mirth, joy, and preference are the emotions/feelings that fuel this magic. Positive emotion charm magic enhances positive feelings or emotions in the caster during casting, and generally is stronger towards ponies or objects that the caster has positive feelings for. Ponies have been known to get addicted to the positive emotion 'high' that happens when using positive emotion charm casting. Positive emotion charms tend to focus more on healing, repair, and empowerment. Positive Emotion Charms are commonly considered 'Harmony' magic by most ponies, as positive charms generally create 'harmonious' results.
Volantus/Charm Casting/Emotion Magic/Negative - Emotion driven charm magic that is impressed upon with negative emotions or feelings. Hate, envy, despair, anger, and fear are the emotions/feelings that fuel this magic. Negative emotion charm magic tends to give casters a feeling of power or thrill that can be addicting, and the morals of unicorns without strong mental fortitude are slowly eroded with use of this magic. The most infamous Negative Emotion Charm caster would be King Sombra, his negative emotion casting removed nearly all of his morals and inhibitions, rendering him remorseless and cruel. Negative emotion charms tend to focus on harming, damaging, and weakening. Negative Emotion Charms are commonly considered 'Dark' or 'Black' magic by most ponies, even though there is no such thing as 'evil magic'.
Volantus/Charm Casting/Charm Binding - Charm binding is when a charm caster binds a charm to an object to provide a certain effect to the object and/or it's surroundings. Bound charms are almost functionally the same as enchantments, but tend to be more flexible. The main differentiation between bound charms and enchantments is that the charm has to have a constant supply of mana, which can be stored in gemstones or drawn from nearby mana sources.
Arcanum/Light Magic - Most spells that are in use today by unicorns are Light magic. Light magic is powered by a unicorn's own mana reserves and is directed by magical formulae.
Arcanum/Shadow magic - Most spells that use Light Magic can be adapted to use Shadow Magic. Shadow magic is powered by external sources, generally the caster has to get permission from a source (the only time that the caster wouldn't require permission is if they were fueling the spell from charged gems). Skilled shadow casters ask the spirit of the night (not the princess, the actual force of nature) to lend them power for shadow spells. There are many different spirits of nature that can be called upon to fuel shadow magic, but the spirit of the night is the most lenient with her power.
Arcanum/Runic Magic - Runic Magic is most widely used in enchantment, but is also used in spell circles and sigils. Runic Magic doesn't draw mana until the effect activates or the initial casting conditions are met. Runic Magic requires the caster to specify where the mana that fuels the spell is coming from, and draw from external sources like shadow magic, or from the spellwriter's own mana reserves. Runic Magic is the most versatile magic to exist, as it is only limited to the spellwriter's knowledge of runes and ingenuity.
Arcanum/Elemental Magic - Elemental Magic is similar to Light Magic, in the sense that it draws from the caster's mana reserves to power the spell and requires magical formulae, but elemental magic requires an elemental affinity from the caster. Every unicorn has an elemental affinity, but most are not in tune with it enough to use elemental spells. For example, an elemental fire spell can create fire directly from the caster's mana (without needing a fuel or air, yes it can be used underwater), while a light magic fire spell requires fuel and air to burn. Elemental magics all follow the same as the example listed above; fueled by mana and doesn't require external factors to sustain.