A state set applies one set of conditions to a block of attributes and/or group sets. Each expression in the block is that attribute's state value under those conditions, so the attributes can't declare inline states of their own.

@Narrow:                                // conditions = {"Narrow"}
    Layout.                             // attributes = {
        Direction: "Row"                //     { declaration = {
        (Align,Justify): "Center"       //           group = {{"Size"}}
    Size.Width: 0                       //           item = {{"Width"}} }
    Corner Radii.                       //       expression -> value -> number = 0 } }
        Top (Left,Right): 20            // group_sets = {
        Bottom Right: 0                 //     { declaration = {
                                        //           group = {{"Layout"}} }
                                        //       attributes = {
                                        //           { declaration = {
                                        //                 item = {{"Direction"}} }
                                        //             expression -> value -> string = "Row" },
                                        //           { declaration = {
                                        //                 item = {{"Align","Justify"}} }
                                        //             expression -> value -> string = "Center" }
                                        //       }
                                        //     },
                                        //     { declaration = {
                                        //           group = {{"Corner"}, {"Radii"}} }
                                        //       attributes = {
                                        //           { declaration = {
                                        //                 item = {{"Top"}, {"Left","Right"}} }
                                        //             expression -> value -> number = 20 },
                                        //           { declaration = {
                                        //                 item = {{"Bottom"}, {"Right"}} }
                                        //             expression -> value -> number = 0 }
                                        //       }
                                        //     }
                                        // }

Without a state set, the same states are declared inline, beside each attribute's main expression. This works, but the @Narrow values end up scattered across the attributes; a state set keeps them together in one block.

Layout.
    Direction: "Column" @Narrow: "Row"
    Align: "Stretch" @Narrow: "Center"
    Justify: "Start" @Narrow: "Center"
Size.Width: 200 @Narrow: 0
Corner Radii.
    Top Left: 0 @Narrow: 20
    Top Right: 5 @Narrow: 20
    Bottom Left: 0
    Bottom Right: 5 @Narrow: 0