This is the visual definition icon responsible for specifying how a Thermodynamic diagram background is displayed. It controls features related to for instance isotherm, isobar and mixing ratio grids.
The macro language equivalent is mthermogrid().
The Thermo Plotting Editor
All of the parameters in this editor come directly from Magics, and are documented on the thermodynamic grid reference page. (insert a link when Magics documentation is updated)