Advanced Systems Format (ASF) Files
All of the objects of the Windows Media Format SDK work with files that conform to the Advanced Systems Format (ASF) file structure. These files are referred to as ASF files in this documentation. For more information about the structure of ASF files, see Overview of the ASF Format.
Sometimes the term Windows Media file is used to describe files created by using this SDK or other Windows Media technologies. It is not accurate to call an ASF file a Windows Media file unless it contains only content encoded by using Windows Media codecs. For more information, see the guidelines for the use of the .wma and .wmv file extensions in the File Name Extension Guidelines section.