An expansion is a new base controlled by its own location manager and with its own brain condition monitor.
So you basically start a new AI brain on any new expansion.
Expansions will be build at free startpoints, and mostly on areas with many mass extractor spots.
Naval bases are also expansions.
I would suggest to only use 2 land and 2 naval expansions for better game speed.
If you use my AI then i would suggest to set land expansions to only 1 and 3 naval expansions.
Onyl thing to mention, the map must have map markers for expansions created by the map autor
or the AI can't buld any expansions.
(If you use the AI-Uveso, then you can enable a option to autogenerate waypoints and expansion markers on any map)