Most lines in a workout represent one step.
That step usually ends in one of three ways:
2km Easy
500m Threshold
Use distance when the point of the step is covering a set amount of ground.
10 min Easy
45 sec Hard
Use time when the point is duration, not distance.
Open steps do not end automatically. You move on by pressing Lap on your watch.
They are useful when you want flexibility in the moment, for example: