Great explanation, thanks Terry
This is referred to as interval training. One swims a predetermined distance a certain # of times. Rest between repeats can be short, medium or long again depending on the goals of swimmer/coach and time of swim season.
Although I have been doing intervals for years, since becoming a mindful TI disciple, the added focus of both SR and SPL has added immeasurably to my appreciation of how to continue adapting gradually.
Take a look at my latest post in "Formula for a faster 1500/1650" for an example of a mindful 64 minute set. With intervals, the possibilities are endless.
Once again, Terry explains it well.