Difference between load resistance and Thevenin/Norton resistance?
In the example you've posted, we see two Thévenin circuits that share a 40-ohm load resistor.  But the question is what differentiates between the "load" resistor and the "Thévenin" resistor.

I've redrawn the circuit below:







simulate this circuit &ndash; Schematic created using CircuitLab

The Thévenin equivalent circuits are enclosed in boxes as shown.

Alone both Thévenin and Norton circuits have open-ended terminals.  Whatever impedance is found across those terminals is deemed the "load" that the equivalent circuit is driving.

For a Norton equivalent, we would see the following circuit:







simulate this circuit