The base mod is open to view and edit (own risk) and is stored in the Good Company folder under \Good Company\mods\base and the changes affect multiplayer.
The base mod describes the behaviour of our content such as competitors, markets, policies, skills, levels etc.
Currently it is not possible to load additional mods, only the base mod exists. Hopefully when offering official modding support multiple mods can exist. Until then you will need to change the base mod.