'Why can't clients write on rpc method variables

so i have a

MoveServerRpc(direction1);

and a

    [ServerRpc]
public void MoveServerRpc(Vector3 Direction2)
{
    player.GetComponent<Rigidbody>().AddForce(Direction2 * 10, ForceMode.Impulse);
}

and the problem is that the

direction1

doesn't get turned into

Direction2

Why?

unity3d networking

Sources

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Solution	Source