Rectangular Caps are much stronger than similar magnets of the same size and type as they have a steel cap/pot on one side of the magnet.

The steel cap redirects the magnetic flux of one of the magnetic poles to the other side of the magnet. This enables both the North and South Poles of the magnet to be concentrated on the same side, thus creating a significantly increased holding capacity. They are considered the most efficient design for holding onto the steel.