This neodymium pot magnet in a steel pot is countersunk. It can be fastened with a countersunk screw. The steel pot substantially increases the holding power of the magnet, but requires full contact with the opposing part, e.g. a steel plate. Magnets in steel pots don't have much effect into the distance, i.e. they are not suitable to attract a metal item from a distance, but they are very well suited to hold on to a piece of metal in full contact.
Pot neodymium magnets Pot magnets, neodymium magnets