Eighty percentage of the world's wealth is with twenty percentage of the people. This holds good for so many other things like population, oil, marketing, production, inventory management etc. If we apply that to pmbok, 80 percentage of the project management benefits come from 20 percentage of the good practices. Another very useful inference is; in pmbok 20 percentage of the concepts gets repeated in 80 percentage of the processes.
A sample list of this 20% of these concepts could read like;
Enterprise environmental factors
Organizational process assets
Project documents updates
Project scope statement
List is incomplete.
If one can master this 20% of the concepts, then the task of understanding pmbok becomes easier.