Each category is a subjective labeling of the various API functions in d3 version 3. The numbers that drive the treemap are counts of API funcions in each category. This is a little like using lines of code as a metric as a higher count doesn’t necessarily mean more important, but it does give a relative sense for the distribution of functionality in d3.
This makes it easy to see the logic in splitting d3 up into smaller modules for version 4.