Hello Duncan, welcome to TalkGraphics.

Doing a search for avi in the Photo and Graphic Designer help brought up the following information.

Make background transparent
Selecting this option will make your AVI have a transparent background, which is useful if you want your animation to appear on top of a movie (for movie titling, for example).
When this option is on, the codec list is reduced to show only the "uncompressed" option, because transparency can't currently be supported using a codec. If you export an uncompressed AVI with transparency, it will import into MAGIX movie editing software and some other movie editing applications with the transparency.
Area to save
Choose whether to save a movie that is the size of your animation page or the size of the animated drawing.
Click OK to export the AVI animation.