Caster is an Occupation in the Shinsekai where users apply mana to temporarily create a certain effect, Spells, in combat and day-to-day situations. Some Casters can even create different applications of mana to create new spells and even go onto create artforms. A Caster's spells are hand written
Being a Caster an be very difficult, people have to establish a process for their spells, which mainly involves generating mana in the body and thinking of an effect that mana should have. For example a person wants to create a fire spell, they generate mana and think of how they want the fire to behave and boom fire. In that sense it's using mana to manipulate reality and create a certain effect, with the amount of mana you put into it will either strengthen the output or make it behave in certain ways.
Because magic is so varied across the universe, spells aren't set in stone but some people can choose to share their magic and hence spells can get around. However, in some instances different spells are created that do the same thing. The process of thinking of how you want something to be effected can be shortened by simply saying the spell name. By doing this, the spell simply just happens in a similar fashion to muscle memory. The Caster has said the spell and their subconscious has done the rest, the only thing they needed to do was to make a conscious effort to generate mana.
Magic so powerful can branch off and become it's own art form, whilst generating a fireball or strengthening a weapon or even replenishing strength are relatively simple and generalised. Something like using necromancy would take a long process of learning how to do and sometimes even require certain conditions to be met. Now continuing on from that necromancy example, it might have a similar process to summoning a spirit with similar conditions so those both get placed in the "Dark Arts".