Life, the universe, and everything

This has been explained in-story fairly clearly, but okay.

Every phenotypical unicorn (that includes Shapies choosing unicorn as their primary shape) has magic to aid them with their special talent. Twilight Sparkle said that much herself. Yet, what would be the point of magic schools like CSFGU if everypony had just what helped them do what their butt says they do? That is a question which I had to tackle, given that I'd portrait a pony in the height of her education.

What I chose is that theoretically, everypony can do every magic. That, however, would once again make any differentiation obsolete. To represent the influence of special talents, everypony acquires a special spell once she gets her cutie mark, which is often a highly powered version, but can be performed without much effort. Also, similiar spells (with a like rune configuration) take less work to learn and perform as well.

Signature spells are usually, as the name implies, unique to a pony. Of course, that is not entirely true. With a fitting amount of effort, anypony can perform any signature spell as those are nothing special intra se, but it is much harder for them than for the original owner of the spell. A seeming exception to this rule is Twilight Sparkle, who apparently does not have a signature spell of her own (or she forgot it), but can copy anypony else's with equally little difficulty. (She has already done Rarity's and Shining Armour's) It is unclear if she genuinely can copy every spell as the owner could cast it or if she simply does not experience strain due to her extensive talent.