The Surface Defaults tab enables you to define a standard set of thickness data that is not attached to a specific toolpath or boundary. You can therefore use it to set default thickness rules for your model.
This is similar to the Surfaces tab, except that the values applied here are default values applied to all new toolpaths and boundaries. Any existing toolpaths or boundaries are not altered by any changes made here.
Set 
— This a visual reminder to indicate that this is a Default setting not attached to any specific toolpath or boundary.