Add multiple fills and strokes to a shape
SVG gives an element one fill and one stroke. A two-tone outline, a sticker border, a keyline over a flat fill - all of them need more paint than that, and the usual workaround is to stack duplicate shapes and keep them in sync by hand.
The Appearance panel makes the paint a list instead. "Add Fill" and "Add Stroke" grow it, rows drag to reorder, and the eye on each row hides that pass without discarding its color. The additional layers appear when you add another fill or stroke.
Every control edits the row you selected
Selecting a row shows that layer's own controls: its color, its opacity, and for a stroke its width, joins, caps and dash pattern. Each layer can take a gradient or a pattern of its own, so a gradient sheen can sit over a flat fill without touching it.
Effects work the same way. Each effect in the Filters panel has an "Apply to" that names the paint layer it acts on, so a glow can belong to one bright stroke while a drop shadow belongs to the shape as composed.
Save and apply a linked graphic style
"Save as Style" in the Appearance panel stores the stack in the Library panel's Styles tab under a name. Applying it to another shape links that shape to it rather than copying - so editing the style later changes every shape using it at once, with no re-application.
Each shape keeps its own core fill and stroke while sharing the additional layers and their order. "Detach" breaks the link and leaves the appearance exactly as it looks.
Export layered fills and strokes as SVG
A stacked shape leaves the editor as its geometry defined once in a defs block and referenced once per paint pass with a use element, wrapped in a group. The browser paints those layers in order using standard SVG elements.
SVG export resolves graphic styles into the layers painted by each shape. Save the .svgs document to preserve the editable link between a shape and its graphic style.