The default subgrade reaction modulus defined in this dialog box is assigned to all new FEM nodes defined using the "FEM mesh/Define nodes" menu item.
If you edit the given value and leave the dialog box using "Done" the newly defined subgrade reaction modulus will be adopted as of this moment for newly defined nodes. If nodes are already defined, they can also be retroactively assigned a default subgrade reaction modulus using the "For all" button.