You can set the scope to count the number of pulses tested through the mask and count errors, but many scopes also do more. For example, if your scope has a persistence mode, then you can see small changes in the pulses. The scope will store each pulse and change color or intensity the more often the pulses attain a given value of timing or amplitude. The persistence-mode display gives you a feel for the pulse's repeatability. Another useful tool in mask testing is a histogram.
Scopes can produce histograms on the amplitude scale or on the time scale.
The wider the histogram, the more jitter in the system. Bit Errors Receivers in telecom equipment sample and digitize the incoming signals, then try to interpret them and reconstruct the original bits. A network's bit-error rate is proportional to the jitter of an incoming signal. If you find that jitter causes pulses to appear outside of a mask, you can look for clues to find the source of the problem. Interference from other signals may cause jitter.