Pages are assigned to regions in an indirect way, by means of their layers.

This is controlled also by the parent zone.

The rule is following: a page can contain only such layers which belong to a region, that contain the page's zone.

How regions change the geographically zoned pages?

a/ Without regions

Split zoned page into the part that is common to all zones (putting all the elements which are supposed to be identical in all zones on a layer, and sharing that layer in pages of all zones), and into the zone-specific part (on the zone specific layer). But, it was not possible to make the an element having a different format, or geometry in different zones -- the only way to achieve it was to make a copy of the text element.

b/ With regions

You can do the same as above, but also to assign more formats, and geometries to a text (one format/geometry for each region), and also to mark parts of the text to be region specific. In that way, you can publish prices in dollars in US region, while keeping them in Euros in the European region.