The occupancy of a datapath at a point in time refers to the proportion of the datapath that contains valid data.
The occupancy of a circuit over the execution of a program is the average occupancy over time from the moment the program starts to run until it has completed.
Unoccupied portions of the datapath are often referred to as bubbles. Bubbles are analogous to a "no operation" (no-op) instructions for a CPU that have no effect on the final output.
Decreasing bubbles increases occupancy. In the absence of other bottlenecks, maximizing occupancy of the datapath results in higher throughput.