With AVIF, multiple spatial layers can be encoded in one frame. A spatial layer may increase the quality or resolution of the decoded image based on one or more previous layers. A layer may also provide the decoded image independently. Moreover, not every layer produces images intended for rendering. Sometimes, decoded images are used only as intermediate images.
Primary sources
1. AVIF - The Full Guide and FAQ2. AVIF AOMedia Specification3. AVIF Wikipedia4. Jake Archibald Article5. AVIF Netflix article