The restricted rate element should e.g. be used to model a kitchen faucet. The valve can be opened or closed, but there is a limit to how much water per time unit you can get out of the faucet, and no manipulation of the faucet will ever make water flow from the sink back into the faucet.
The model contains a dissipative loss of enthalpy (but not mass) to the environment. Mass is always conserved. By attaching an R element to a 1 junction, we guarantee that the mass flow is not affected at all by the dissipative term. Only enthalpy is being dissipated.