The "VersionType" merely restricts the version string that may be used with XML documents based on this schema. LAYERS AND STYLES A NamedLayer is a layer of data that has a name advertised by a WMS. A NamedStyle is used to refer to a style that has a name in a WMS. A UserLayer allows a user-defined layer to be built from WFS and WCS data. A UserStyle allows user-defined styling and is semantically equivalent to a WMS named style. A FeatureStyle contains styling information specific to one feature type. This is the SLD level that separates the 'layer' handling from the 'feature' handling. A Rule is used to attach property/scale conditions to and group the individual symbols used for rendering. A "Graphic" specifies or refers to a "graphic symbol" with inherent shape, size, and coloring. A "Mark" specifies a geometric shape and applies coloring to it. A "Stroke" specifies the appearance of a linear geometry. It is defined in parallel with SVG strokes. The following SvgParameters may be used: "stroke" (color), "stroke-opacity", "stroke-width", "stroke-linejoin", "stroke-linecap", "stroke-dasharray", and "stroke-dashoffset". Others are not officially supported. A "GraphicStroke" defines a repated-linear graphic pattern to be used for stroking a line.