REGIONS are build volumes of the part/support set-up that can be printed with varying print parameters.  For instance, lets say there is a scenario where the single part needs a different set of print parameters defined by its functional importance by design engineering. The manufacturing engineer can address this by identifying these print volumes as REGIONS and apply varying print rules.  There are 2 main uses of REGIONS. One is to define varying print rules for different sections on the same part and, two, for defining support structure with user defined CAD geometry. This capability is available through BUILD LAYOUT command as seen in the pics below.