The server.cs is exec'd when you start the server with the package selected.
The client.cs is exec'd when you start Blockland.
For using multiple .cs files, you need to use the exec("./filename"); function in server.cs or client.cs.
An example mod for using client.cs would be auto-jet, something that uses what you already have to do something unique.
An example mod for using server.cs would be something the server needs, no person can just bring weapons onto a server, they must be pre-existing. This is why server.cs is there, to make the weapons, tools, vehicles, events, and other scripts on the server for the clients to access.
Hope I explained that well.