"Mission data. The server needs scenarios for all missions, task forces and events in order to be able to play them for you.
Costume data. The server needs to render other entities in the world for you."
Both of these were not handled server side, random instances such as dev created invasions were plotted and started server side however all the required programming IE maps, missions, characters, story and related mechanics are in the .piggs files. the server did run hit calcs and relay the actions to any other available parties.
Edit: character information was saved server side though