Paint glossiness is determined by the chemistry of the paint. Paints that are “high-gloss” have a larger ratio of resins and less pigment volume concentration (PVC). Flat paints, on the other hand, have a much larger ratio of pigments and a lower ratio of resins. This explains why painting with glossier paints often requires a second, or even third coat, while flatter paints may cover adequately with a single coat.
Although homeowners are usually content to know that flat paint doesn’t reflect much light and that high-gloss paint reflects a lot, the industrial paint industry actually has fairly precise standards for how to categorize different paint finishes, based on how much light they reflect.
While in the broader residential paint industry there are six different levels of paint finish, be aware that not all paint manufacturers offer all six. And they may have slightly different terminology for the same finish.
For example, one manufacturer’s “glossy” paint may be the same as another’s “hi-gloss enamel.”