external canister filters

I am busy examining the specifications for various external
power/canister filters, and was hoping someone could explain what the
specification "Delivery head 1,50 m/WS" means. I guessed it was to do
with the distance between the filter base and the surface of the water
in the aquarium, however for the same filter this figure above is for,
there is also listed the specification "The distance between water
surface and base of the filter should not exceed 180 cm"

Hence I am at a loss...any suggestions??

Thanks

Grant