An elastic surface foundation represents an elastic support of all 2D elements of a surface. As a support or spring, you can define the spring constants Cu,x/Cu,y/Cu,z. The direction of the support refers to the local surface axes x, y and z.
The unit kN/m³ (kip/ft³) describes the force, which — for example, in the case of Cu,z — is required to shift a surface of one square meter by one meter in the z‑direction (hence m³ / ft³).