Android Netrunner Comprehensive Unofficial Rules Wiki
See also the region category.

Regions are special subtypes of upgrades. They are limited to one per server, and as such tend to be more powerful than other upgrades.

Page 13, Column 2, Paragraph(s) 3, Core Rule Book
Note: The Corporation can only have one upgrade with the region subtype installed per server or server root, as listed in the text box of these cards.