We will repeat the same as for the classical use example
and show its tree in case it is hidden, with the menu Data->Data tree
a frame on the botton of the tree will allow to select the material: Air and expect to be assigned to groups of entities.
In general a recommendable workflow is to create first the groups of entities and to apply in a second step the calculation properties to these groups, but for simple cases it is possible to create automatically the group during a selection, pressing the Select button
with the menu: Utilities -> Layers and groups you can open the groups window, and see that the selection has created a couple of new groups, with automatic names based on the data tree items.