Not sure if I can model soil stiffness on an area using Robot.

I know I can click on the "support" icon then click on "New" then "Support Definition" appear.

So I click on "Elastic" menu ? then click on 'Elastic Coefficient" Button to define the building soil, then choose "Raft foundation" with dimension 1mx1m.
Then choose the soil layer, I have Kz (kN/m3) for soil stiffness. Is it this value apply for the node or area I click on the model ?

I try this and it seem applty on the edges and nodes not on the whole slab area

Thanks.