Simply put, regions are sites within sites. Creating multiple types of buildings within a single site can be achieved with the regions tool.
For example, by defining regions within an existing site you can have a tower and gurban style building, with a pond & park, with a parking lot on a single site.
Check out the Knowledge Base Articles related to Regions
Adding Regions to a Site
Adding Regions to a specific level
Change a Regions Preset
Dependent fill regions
Reorganizing & Renaming Regions
Resizing Regions