Polished plaster finishes build colour in thin layers and then burnish the surface, so light both reflects off a smooth outer plane and scatters within the layers beneath it. The result is a sense of depth that a single-plane paint film cannot produce: the colour appears to sit inside the wall rather than on it, and shifts as you move past it. This is also why these finishes photograph poorly — a still image flattens exactly the depth and movement that define them.
These finishes are built from several thin layers and then burnished, which compresses and smooths the outermost layer while leaving the ones beneath it slightly translucent. Light therefore both reflects from the polished plane and scatters back from within — two returns from different depths at once. That combination is what reads as depth, and it changes as the viewing angle moves, which no single photograph can convey.
Matters most where a wall is meant to be a surface in its own right rather than a background — a feature wall, a bathroom, a hallway. Matters least in flatly lit spaces, where there is no directional light for the burnished surface to work with.
Assuming the finish can be judged from an image, or specified like paint. It is an applied craft finish whose result depends on the applicator and on how many layers are built up, so two walls in the same product can look quite different.
Practical advice
Judge these finishes in person, on a large sample, under the room's own light and from more than one position. They are specialist applied finishes rather than a paint you roll on, so treat any colour reference — including this site's — as indicative of hue only, not of the finished effect. Application, substrate preparation and product-specific method all materially change the outcome.
Always confirm with a physical sample in the actual finish before committing to a large surface — see methodology.