Excessive ringing and overshoot can be caused by a line transformer on the output device. In electrical signals, ringing can also indicate that the transmitting device is emitting excessive conducted EMI through the cable. So even though the pulse fits within the mask, the oscilloscope can indicate potential transmission problems. Masks also indicate if the transmitted signal has sufficient amplitude to arrive at its destination within tolerance. Each standard defines an acceptable range of amplitudes.
European standards define amplitude in absolute voltage levels.
For example, a 2.048-Mbps pulse has a nominal amplitude of 2.37 V if traveling on a coaxial cable and 3 V if on a twisted pair. If the amplitude is too high, the line may not be properly terminated. In contrast, North American masks don't define absolute voltage levels. Instead, they define relative amplitudes between the high and low voltage levels of a pulse. To minimize overshoot, undershoot, and amplitude problems, you need to match the impedance of your scope to the transmission line.