In Photoshop, what does adjusting fill opacity do?

Adjusting fill opacity in Photoshop specifically affects the transparency of the contents of a layer while leaving any layer styles—such as strokes, shadows, or glows—at their original opacity. This means that if you have a layer with a fill color and you reduce its fill opacity, the color will become more transparent, but any effects applied to the layer, like a drop shadow, will remain fully visible.

For example, if you have a solid colored shape with a drop shadow and you reduce the fill opacity to 50%, you’ll notice that the fill color of the shape is semi-transparent, while the drop shadow remains at full strength. This functionality is particularly useful when you want to create more complex visual effects or blend layers without impacting the visual properties of layer styles.