Soft-in-plane is a general helicopter stability and control term, not limited to one manufacturer or country.
Soft-in-plane refers to a rotor head system where the lead-lag natural frequency is less than or equal to the shaft rotational frequency. For a fully articulated head (soft-in-plane) the lead-lag frequency is typically 0.2 to 0.3 the shaft rotational frequency (these numbers are from "Principle of Helicopter Aerodynamics", Leishman).
Stiff-in-plane is where the lead-lag frequency is greater than the shaft rotational frequency.
A general discussion of the term "soft-in-plane" can be found in"More Helicopter Aerodynamics", R.W. Prouty.