How is RPC used?

Unity Engine
1

I have never used it, and the documentation at http://docs.unity3d.com/Manual/UNetActions.html doesn’t make much sense to me.
I have used commands and [SyncVar] to manually change position of my players on all clients.

My main goal here is to send a sbyte[,] from the host player to any client that joins (only just once when they join).
I could change this to a string if needed.

The way I imagined this working is that in the start() method I could call a command and it would tell the server to send the array back to them. I have no idea how this would work though.

I had been following a tutorial to set up the position synchronization, and I also have a question about the following code:
Code

[Command]
    void CmdProvidePositionToServer(Vector3 pos){
        //Debug.Log ("syncing position");
        syncPos = pos;

    }
    [Command]
    void CmdProvideRotationToServer(Quaternion rot){
        //Debug.Log ("syncing rotation");
        syncRot = rot;
    }


    [ClientCallback]
    void transmitTransform(){
        if(isLocalPlayer){
            if(Vector3.Distance(myTransform.position, lastPos) > posThreshhold){
                CmdProvidePositionToServer (myTransform.position);
                lastPos = myTransform.position;
            }
            if(Quaternion.Angle(myTransform.rotation, lastRot) > rotThreshhold){
                CmdProvideRotationToServer (myTransform.rotation);
                lastRot = myTransform.rotation;
            }
        }
    }

Why is it that you cannot just set the synced variable in the transmitTransform() method?

2

transmitTransform runs only on clients (and only on local player), but commands from it are executed on server.
In Cmd methods server may then send those positions back to other clients to keep them in sync using Rpc methods for example.