As your computer renders an image, it divides the image into small areas called buckets.
This image shows a model being rendered with PhotoView 360, using Network Rendering. The orange squares are buckets that the Coordinator computer is processing. The computer has four cores. Each core is processing one bucket.
The blue squares are buckets that the Client computer is processing. The Client computer has four cores, and has been assigned a Client Workload of 200%, so each core is processing two buckets at a time, for a total of eight.