Rise time is usually referred to in digital circuits. The shorter the pulses the more important the rise time has become. It is measured as the time from the beginning of a pulse to the 90% point of the pulse height. Circuits need to be able to have good RF characteristics to pass these fast pulses. Rise time is an important parameter that needs to be accounted for. Circuits not capable of handling fast rise time pulses would effectively swamp out the digital pulses.