This property modifies how a HandFireArms object fires depending on the soldier's stance (standing, crouching, prone). It modifies the deviance (in degrees); typically, a prone soldier is more accurate than one standing, so has a lower deviance modifier. The parameter positions mean:
x - standing y - crouching z - prone
See the related properties for more information.
For Vietnam, there is a fourth argument, which is the devModZoomedFactor.
Source and more details