Material Editor > Make/Play/Save Preview flyout > Make Preview
The Create Material Preview dialog is displayed when you click Make Preview to preview an animated material. By default, the preview is saved in the \previews subdirectory with the name _medit.avi. You can use Save Preview to give the preview a different name so it won't be overwritten the next time you use Make Preview.

Active Time Segment—Renders the active time segment.
Custom Range—Renders a custom range from the start to the end frame you specify in the spinners below.