The list of properties is put together as follows:
Enable or disable the property for all selected faces. The checkbox is checked when all selected faces have this property.
Indicates how many of the selected faces have this property.
Hover the property name to find out more about the property.
Select (sel) button:
Click to select/deselect all faces with this property.
Sets the texture number for all selected faces.
-1 if numbers of selected faces don't match.
Set: Displays the material of the currently selected face. Select a material from the list to apply it to all selected faces. The select (sel) button selects all faces with the same material.
Select: Select a material from the list to select all faces with the same material.