Division basis is a region field naming the kind of division the region belongs to. A city is often divided several ways at once, and the resulting areas overlap on the ground while answering to different authorities. This field says which of those schemes a region came from.

The value applies to the whole set of sibling regions, not to one of them. Every quarter of one scheme carries the same basis, because the basis is a property of the division, not of the individual area. A region whose scheme is unknown leaves the field empty.

It does not say how far down the hierarchy a region sits, which is the region group, and it does not say what the unit is called, which is the region type. Two regions can share a group and a type and still come from different divisions, which is exactly the case this field separates.