From the page you referenced:

MICROSOFT OPEN LICENSE BUSINESS

Microsoft Open License is a software volume licensing program designed for corporate, government, charity, and academic customers who order as few as five licenses. After the initial order, customers can benefit from volume pricing for all licenses for the remainder of the term of their Open License authorization number.

Open License Business offers savings on estimated retail prices by placing an initial order for five or more licenses or one server processor license. With Open License Business, customers can combine any set of Microsoft products to qualify for the five-license minimum.