Greater: A Greater Scholar is one who concentrates on a single sphere. To become a Greater Scholar, pick the same option when asked to choose your major and minor sphere at creation. As a Greater Scholar you will have access to the most powerful spells of your chosen sphere.
Mixed Major/Minor: Instead of becoming a Greater Scholar, you can choose to mix your spheres at creation. This provides a more versatile spell list and opens up all sorts of interesting roleplay possibilites, at the expense of the more powerful spells a Greater Scholar gets access to. What major sphere you pick determines your title in the who list and what spells you get the most of. For instance, a Water/Spirit Scholar will get mostly spells from the Water sphere, with some Spirit spells mixed in.