I have no idea what a sanctum does either, but I can make something useful for a sanctum to do. How about this:
Arcane Sanctums
An arcane sanctum is a practitioners clean space. The place where he will do most of his most complicated spell weaving. Of course, certain preparations must be made to make such a place more suitable for the task. Wizards are fond of using special incense known to drive away malign entities, many sanctums include circles of protection and very powerful wards.
For every three ranks a Arcane Sanctum has, you may give it a scene aspect, which you may tag freely for any use of magic. For example:
Heather uses her arcane sanctum as a place to work her powerful divination spells. Her sanctum is a basement with soft lighting, and especially clean floors, it has a rating of 6. She has put a scrying pool in the center of the room, shaped as a perfect circle, and filled it with clear, pure spring water from a jug she keeps on the shelf (Aspect: Crystal Clean Scrying Pool). She has also set up wards which (unlike conventional wards, which purpose is to keep things out) draw in magic from the outside (which is then purified by the ward), and pool it into the room (Aspect: Magically Charged Environment).