The Pulse shape is used to modulate the current value based on location to the bar or beat. It is similar to the Sync shape -- in broad terms, think of Sync as a generator and Pulse as a processor: Pulse takes whatever data is fed into it by the preceding shape and multiplies it by a value according to its internal shape and the current bar/beat location.
The Pulse shape can have one or two internal shapes. The first is used for the bar offset (and the beat offset, if no beat shape is present but the Beat Amount is not 0), the second shape is used for the beat offset.
Pulse is an internal shape used to build other shapes, and is available only in the shape editor. To understand how it works, read the Pulse Stepper description.