They're very different! ConnectionService is a server side class, one you use when sending a message from the server. DarkRiftAPI is a client side class so one you use as a client. If you try and send using DarkRiftAPI from the server you'll get an exception because you're not connected as a client to anything, similarly using ConnectionService (or any other server side DarkRift class wont do anything in the client because you're not the server. This is split up a lot better in DarkRift 2 so should avoid future confusion!