The Ext.NET license is appropriate if you wish to develop, sell and/or otherwise distribute closed-source proprietary applications which include Ext.NET.
Ext.NET is licensed on a per developer basis. Each individual who uses Ext.NET for development purposes must obtain an Ext.NET license to do so.
The Ext.NET license also grants a royalty free runtime license for you to include Ext.NET in your product and sell an unlimited number of copies of your product without a royalty fee, subject to the terms of the License Agreement.
One Ext.NET license is required for each developer working directly with Ext.NET in an IDE, such as Microsoft Visual Studio, Microsoft Visual Studio Express, or similar.