Server only / Forwarded event messages: The cog will forward event messages to the server. Messages that are sent to all cogs on a client computer (such as startup) will not be forwarded - they will still be handled locally. But messages like damaged, entered, and created will be sent to the server to be handled.
Some events, such as created, occur on all computers in a game (global events), but when clients forward this event message to the server, the server will only see it as one message. And the created event need not occur on the server's computer for him to recognize the forwarded message.
Synced effects: Normally, a cog will broadcast information to the other computers in a game to sync certain events - like the creation of a thing. But note that not all events are meant to be synced or work on other players' computers.