Microsoft Knowledge Base Article 889713 contains the following summary and introduction:
Microsoft Windows Server 2003 can be obtained through five channels. These channels include the following channels: retail, evaluation, volume licensing programs, OEM, and checked build. You can use several methods to determine the channel that your software was obtained through. Additionally, this article discusses differences that exist between software that is obtained through different channels. It discusses Windows Product Activation and the Product IDs that are generated when you run the Setup program. This article discusses how to use the Product ID to determine the Microsoft Product Code and to determine the channel that your software was obtained through.
INTRODUCTIONThis article describes how to determine the channel that your copy of Microsoft Windows Server 2003 was obtained through. This article also describes the characteristics that distinguish the product when it is obtained through these different channels. The channels include the following:
• Retail • Evaluation • Volume licensing programs • OEM • Checked build