A group set is a group declaration of attribute nodes. Group set declarations contain a group without an item (traversals are still valid), and the attributes in the set are declared within an indented block.

Inner attribute declarations carry only their items. A traversal can apply to the whole set, but not specific attributes within it.

Border.                     // declaration = {
    Thickness: 8            //     group = {{"Border"}} }
    Fill: Theme/Gradient    // attributes = {
                            //     { declaration = {
                            //           item = {{"Thickness"}} }
                            //       expression -> value -> number = 8 },
                            //     { declaration = {
                            //           item = {{"Fill"}} }
                            //       expression -> reference = {
                            //           traversal = {"Theme"}
                            //           item = "Gradient" } }  }

The group set above declares the same attributes as the following, only the group set doesn't repeat the group name.

Border.Thickness: 8
Border.Fill: Theme/Gradient