Minor Arcanas are what magi mean when they say "Arcanas" as a general whole. While historically a sub-category of Major Arcanas, today the Majors are considered mostly obsolete and entirely too broad to describe anything. When you speak of the Arcanum, the system of magic-categorization, you are foremost refering to the Minors.

Minor Arcanas are a little bit like a master's degree or apprenticeship in your chosen field of study or work. Learning a Minor Arcana is at least five year's worth of dedication and hard work - something that usually ends in most magi only learning one. A Minor Arcana always have one or very few reigning masters of its craft; this is what a Magus is. A Magus is a teacher and a mentor, but also someone who has fully grasped the teachings of that particular Minor.

Because learning magic is rarely merely learning how to cast a particular kind of spells. Most Minors are intimiately associated with the purpose for which they were created, which reigns from baking to spellfighting to the theory of the Stellaeri's materium. An easy way of thinking about it is that if you ask someone "What's your Arcana?" it's the same as asking "So what do you do for a living?".

Most magi expect to only have one Magus in their entire life, and that one Magus is to be chosen with care and forethought. They will inevitably become something of a third - or in Azurites' case, the only - parental figure in a mage's life. Together with the spells and rituals of the Minor's particular magic, the Magus is expected to bestow life wisdom and more practical skills alongside - such as how to make a dough rise or get your paper published in the medical journals.

When choosing a Minor Arcana for your character, it's the equivalent of choosing their profession or their DnD class. It's worth to take into consideration that if they've managed to complete their training, they are a Magus in their own right - and so, they are expected to take on students of their own. If they are still neophytes to a Magus, that Magus should be an important part of their life, NPC or not.