The stated holding force applies in ideal conditions without a safety factor, on a 15mm thick iron plate of the material Fe35.
ED's is a percentage measure of the connection time. If the magnet are supposed to work shorter (lower ED), you can increase voltage and get more power. Lower voltage gives lower holding force.
INSTRUCTIONS FOR USE
The holding force (necessary to release the electromagnet in a perpendicular direction from the iron surface) depends on the following factors:
The gap between the electromagnet and the iron surface. The holding power is most optimal if both the surface of the magnet and the iron have direct contact, with as good surface evenness as possible.
The thickness of the iron surface. The electromagnetic force is most optimal if the thickness of the iron load exceeds 15mm.
Material. The material must be ferromagnetic. Good ferromagnetic material, from better to worse in descending order: Fe 35, steel 60, moldable cast iron, gray iron and high speed steel.
Voltage. The voltage should not be less than 90% of 24DC.
Here we can see how the magnetic field is affected when the iron plate is less than recommended.