Generally our magnetic cup is composed of an outer round cup steel and a ring ceramic magnets inside with a hole in the centre, which has been broadly applied into holding, fastening and handing obejects, or used in some home kitchen application.
The inner magnet of cup magnets could form a special magnetic circle with the the housing steel when putting it on an ferrous material surface, which is much be strongger than the the magnet self can do. At the same time, the outer cup steel can prevent the inside magnets from damaged or crushed. In some of applications, it can be filled with Epoxy between the cup steel and the magnet to stop some iron dust adsorbed into the gap. Or it will reduce the magnetic force of this round base magnet after several using.
Also if you need the holding force better, you can change the ferrite magnet into ndfeb magnets.