Hello,

RL is a resistor and inductor circuit series. When a DC voltage is spontaneously applied to the circuit, the voltage decreases with time. On the other hand in the resistor and capacitor circuit series when DC voltage is applied to the circuit the voltage tends to increase with. The time variation in these two occasions is called ' Transient time'. As a result of these behaviour of RL please RC In The RL RC diagram. The two circuits are used for pulse generation or excitation circuit application in different forms.

Hope this helps, Good luck.