The WPSSO Core Premium plugin is licensed based on the WordPress Site Address URL (see the WordPress General settings page). Typically, development, staging, and production sites, for example, would require 3 licenses. In the spirit of helping our fellow developers, we are offering a free development license for every license purchased.
If you purchase a license for your-domain.com (or www.your-domain.com), you can also register dev.your-domain.com using the same Authentication ID. This allows developers to purchase a license, develop a customer’s website, and then change the WordPress Site Address URL value upon delivery, without having to purchase an additional license for the production website.
The developer special also applies to multisite licensing — if you have 2 licenses for your-domain.com/siteone/ and your-domain.com/sitetwo/, you could also register dev.your-domain.com/siteone/ and dev.your-domain.com/sitetwo/ using the same Authentication ID. The installation order is not important – you can register a development site first, and the production site afterwards.
In summary, your-domain.com, www.your-domain.com, and dev.your-domain.com are all considered equal for licensing purposes (they share the same license). All other WordPress Site Address values, like test.your-domain.com, staging.your-domain.com, etc., must be licensed to use the Premium version.