In the Citadel, magi divide different spells and spellwork into a system called the Arcanum. The Arcanum consists of three hierarchical categories in descending order: Discipline, Major Arcana and Minor Arcana - but most people only ever need to concern themselves with the Minor Arcanas. The Arcanum is based on the Seven Sorcerer’s systematic attempt at dividing magic into categories, and while widely used and respected, it’s important to understand that this is all it is: A system. It holds no real sway over the way magic operates in any other way than that the mage believes it does.
Each Discipline is to be viewed more as a branch of magic philosophy than it is a type of magic. They were each founded by the Seven Sorcerers of old, and correlate in turn to the kind of magic each Sorcerer performed. Azyn of the Storm and Acre of the Forge were elemental mages, and thus they ruled Earth and Sky in the Imperium Discipline. Atra of the Torn Cloak and Ade of the Scales warped mind and space with the Major Heart and the Major Space Arcana in the Mystikos Discipline. Aurea of the Clocks allegedly dedicated her life to the study how to craft a clock that could turn back time, and did so with the Major Construction Arcana. Argent the Uncrowned was concerned with matters of beauty, and named his contribution to the Arcanum the Major Artistry Arcana. They both worked within the Magnum Opus Discipline. Lastly, Alba of the Bone sought to understand and conquer death with the Major Birth and Major End Arcana in the Memento Mori Discipline.
The Imperium Discipline state that the way to practice magic, is to rule and conquer it; to bend the Stellaeri to your will. The Mystikos Discipline preaches that you should ponder the nature and strive to understand the depth of the Stellaeri, and only in doing so will you truly conduct magic. The Magnum Opus Discipline believes that time and careful work, to truly dedicate yourself to your spellwork, is what makes great magi. And lastly, the Memento Mori Discipline, which cares little for the nature of magic and more about the nature of life and death; believing that only in the depths of decay and birth, is there any truth.
The Major Sky Arcana is generally concerned with matters of the heavens, such as air, light, wind, rain and lightning. The Major Earth Arcana is instead focus on what rests below, such as soil, trees, metal, plants and animals. The Major Space Arcana seeks to bend and reshape space itself, while the Major Heart Arcana does the same, but to your mind and desires. It’s probably for the best that most Major Heart Arcana spells are lost. The Major Artistry Arcana is about enhancing what already is, jewels and faces alike. The Major Construction Arcana contains all that which is built anew, and then assembled; most automaton and magic-mechanics can be found here. The Major Birth Arcana concerns all living matter, albeit usually human such; so flesh-crafting and transmutation is the most common practice. The Major End Arcana meddles with death, such as spirit-whispering and necromancy.
However, as time passed, the students of the Seven Sorcerers developed and improved upon their old teachers’ magic. In the end, the Major Arcanas became too narrow to include all the spellwork in the Citadel, and the solution became Minor Arcanas. Originally, Minor Arcanas were closely related to their Majors, but today there is such a multitude of them that it can be difficult to pinpoint exactly how a Minor Arcana resemble their Major.
In the present-day Citadellian society you use Minor rather than Major (because majors are simply too huge subjects to say anything relevant about a mage) Arcanas in order to describe your A. Education, and B. Profession.