When you have answered all the questions in a knowledge area, and the three progress bars at the top of the knowledge area page all show 100%, the option to COMPLETE KNOWLEDGE AREA will appear just below the progress bars.
Completing a knowledge area locks-in your answers. An assessment cannot be complete until all of the knowledge areas are marked as complete. This should be done when the answers have been reviewed and the person the knowledge area is assigned to is confident that the answers are as accurate as they can be.
If you find that you need to make changes after the knowledge area is marked complete, the assessment owner has the option to mark the knowledge area INCOMPLETE. This is done from the COMPELTED tab on the assessment page, using the knowledge area card menu (three vertical dots).