MST Inter and Intra region Concept?

In the above diagram we have 2 mst region region 1 and region 2 . An mst region is group of switches with consistent mst configuration .In every region we can create instances and can have different root brigdes for different instances but in every region we have a  instance   instance zero ie is created  by default when we configure mst on a switch all the vlans  that are not mapped to an instance manually they get automatically mapped to the zero instance.In every region root for the instance is zero is called  the regional root like in the diagram sw1 and sw2  are the regional root for the region 1 and 2. When stp election occurs between the regions one of the  regional root becomes the root of all mst regions ie called cist root  and election occurs accordingly.