Knee measurements are used any time a garment fits close around the knee. I mostly seem to use them for making gathered bloomers and knickers.
The Knee is measured over the center of the patella (knee cap). Stand normally, with your weight distributed evenly over both feet. Now, with the tape around the knee on your dominant leg, squat down. Allow the tape to slide to accommodate your bent knee. Stand up again. The tape should be loose around your knee now, but that’s ok – you want a knee measurement that gives you room to move.