The holding force indicated is only an approximation, and should give you an idea of the strength of the magnets. We calculate this as a pure tensile force against an 8mm-thick steel plate. The weight which is necessary for removing the magnet from the plate is calculated. Other holding forces may arise from other applications, e.g. vertically on a wall, primer coated, thin or uneven, or other environmental conditions.
You should not rely on this information, but always check if the holding force of the magnet is sufficient for your purpose.
When calculating the holding force, the flux density, the field strength, the volume and the distance between the poles are crucial. Other environmental factors such as temperature, etc., are taken into account. The attractable material, as well as the surface, texture, volume, etc. thereof, must also be considered. This turns the calculation of the holding force into a complicated procedure.