Internal resistance of non ideal current and voltage sources
Think a bit for yourself. Would a resistor in series with a current source change the current to the load at all? Would a resistor in parallel to a voltage source affect the voltage on the load? The reason for those resistances is to model that the sources are not ideal. In an ideal voltage source, the series resistance is zero. In an ideal current source, the parallel resistance is infinite. In a real voltage source, part of the voltage is wasted in its series resistance. In a real current source, part of the current is wasted going thru its internal resistance (which is in parallel, so that current is 'stolen' from the load).–