As you learn spells, you will most likely begin to favor one type over others. Some mages go beyond this, and begin to specialize in one school of magic. A specialized mage is one that is exceedingly proficient within that school, and as such can cast above and beyond the typical limitations.

When you only have spells in a single school, you are able to utilize the extra bonuses for each spell in that school. This is lost if you take more than one school of magic.

For example, the spell Magic Missile has the following specialization. The number of missiles fired is increased by 1, as is the damage dealt. This would mean that a mage that is specialized in evocation will fire 2d4+2 for a level 1 Magic Missile.

When you select your first spell, this is said to be your primary school. Subsequently, all spells that you take in that school are unaltered and cost the same as always.

If you later take a spell that comes from a different school, this is said to be a secondary school of magic. Each spell from this school will cost 2x Research. This trend continues upwards for each school of magic invested in.