Relevant to Blender v2.31
As said before Blender is extensible via a plugin system, and two kind of plugins may be found: Texture and Sequence plugins.
Sequence plugins works on strips in a way similar to that of conventional
CROSS etc. operation. You must
have at least a strip
selected and press SHIFT-A>>Plugin or
Menu entry. This opens a File Selection Window
in which you can select the desired plugin.
Plugin functionalities varies so much that it is not possible to describe them here. Differently than Texture Plugins Sequence Plugins do not have a Buttons in any Button Window, but their parameters are usually accessed via NKEY.