Extensions in Maxwell are dynamic libraries, that are loaded on demand. They have extension .<platform>.mxx and are typically stored in the extensions folder inside Maxwell applications folder.
There are several extensions type that group extensions with the same functionality, and that need to implement some specific methods depending on type. All extensions types inherit from class CbaseExtension that handle the general interface with Maxwell, and the methods all extension must implement in order to work correctly. There are currently 3 geometry extension types that can be created, there are plans in the future for more extension types for other different needs and not only geometry, but they are in the works and not fully supported yet.