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The file extension ASF stands for  Advanced Systems Format.   Originally it was called  Advanced Streaming Format, Active Streaming Format.   This file format is actually a  Microsoft proprietary digital audio/digital video container format, especially meant for streaming media.

The ASF format does not specify how media (audio or video) should be encoded.  It just specifies the structure of the video/audio stream.  You might find this similar to the function performed by the QuickTime, AVI, or Ogg container formats.

One of the objectives of ASF was to support playback from digital media servers, HTTP servers, and local storage devices such as hard disk drives.

The most common filetypes contained within a file extension ASF  are Windows Media Audio (WMA) and Windows Media Video (WMV). Note that the file extension abbreviations are different from the codecs which have the same name. Files containing only WMA audio can be named using a .WMA extension, and files of audio and video content may have the extension .WMV. Both may use the .ASF extension if desired.

ASF files can also contain objects representing metadata, such as the artist, title, album and genre for an audio track, or the director of a video track, much like the ID3 tags of MP3 files. It supports scalable media types and stream prioritization; as such, it is a format optimized for streaming.