A number of my clips are animated 3D objects. They appear to have a black background, but the description may say "embedded alpha channel".
This means that instead of tacking on an "alpha channel" or "luma matte" on the end of the clip, the clip was encoded using the Apple PNG codec so that the black background pixels are automatically removed for you by your NLE software.
That means when you place the clip on top of another clip, the clip below will "show through" wherever the background has been removed to only reveal the object itself.
This works with Adobe Premiere, Adobe After Effects, Sony Vegas, Ulead/Corel VideoStudio and many others. If you have questions, please contact me and I'll try to help.