Yale claims compliance with ANSI B56-1:
Cluase 7.17 Parking Brake System Performance for Trucks Up to and Including 50 000 kg Capacity
7.17.1 A parking brake (or mechanism) that may be a part of, or include the service brake shall be provided and be capable of holding the truck on the maximum grade that the truck can climb with rated capacity load, or on the following grade, whichever is the lesser:
(a) order picker, high lift: 5%
(b) order picker, low lift: 10%
(c) motorized hand and hand/rider: 10%
(d) sit-down rider, electric or combustion powered: 15%
(e) stand-up rider, electric or combustion powered: 15%

The inference is that 10% is the maximum slope fpr the Yale MPB040-E
But I would strongly recommend you do a thorough risk assessment of any operation on a 10% slope including consideration of loss of the load and or loss of control