Apple Compressor 2 - Manual (Page 120)

120 Chapter 9 Creating MPEG-1 Output Files
About System and Elementary Streams
You can use Compressor to create system or elementary MPEG-1 streams.
System Streams
System streams, also known as multiplexed streams, combine the video and audio
components into the same file. This type of stream is most often used in web applications.
Compressor automatically creates a system stream when you enable both the Video
and Audio tabs in the same setting.
Elementary Streams
With elementary streams, there are separate files for the video and audio components.
Elementary streams are required by some applications, such as DVD Studio Pro.
Compressor automatically creates an elementary stream when only the Video or
Audio tab is enabled, but not both. To create elementary streams of both the video
and the audio, you must use two settings—one with the Video tab enabled using the
file extension of .m1v, and a second with the Audio tab enabled using the file
extension of .m1a.