Masks, Compositing, and Smart Objects
Masking concepts
Know layer masks (grayscale, non-destructive, black hides, white reveals, gray partial), vector masks (path-based, crisp edges), clipping masks (a layer clipped to the one below, useful for adjustment layers confined to a shape), and alpha channels (saved selections). Understand that painting on a mask with black hides and white reveals, and that masks can be disabled, linked, or applied.
Compositing workflow
Compositing combines images using masks, blend modes (Multiply, Screen, Overlay, Soft Light), and selection refinement. Select and Mask is central for hair and edges, with edge detection and output to mask or new layer with mask. Understand layer groups and pass-through vs normal group blending. Photomerge and Auto-Blend Layers appear in panorama and focus-stack questions.
Smart Objects
A Smart Object embeds source content and preserves it through transforms, so repeated scaling doesn't degrade quality until rasterized. Smart Objects enable Smart Filters (editable filter settings), Replace Contents, and linked vs embedded variants. Know that rasterizing a Smart Object makes it a normal layer and loses editability.
Worked rule of thumb
If a stem asks how to apply a filter that can be re-edited later, the answer is to convert to a Smart Object then apply a Smart Filter. If it asks how to confine an adjustment to a shape, use a clipping mask.
Traps
A layer mask uses grayscale values, not color; a vector mask uses paths. Clipping masks affect only the layer directly beneath in the clip stack.