Nystagmus is characterised by the combination of 'slow phases', which usually take the eye off the point of regard, interspersed with saccade-like "quick phases" that serve to bring the eye back on target.
During such gaze saccades, first the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex (VOR) causes the eyes to roll back in the head to keep gaze on the target.
In this case the categorization is binary: Either a given saccade is an express saccade or it is not.
The latency cut-off is approximately ~200ms; any longer than this is outside the express saccade range.
By moving the eye so that small parts of a scene can be sensed with greater resolution, body resources can be used more efficiently.
Saccades are one of the fastest movements produced by the human body (blinks may reach even higher peak velocities).