1/4 Stop Silk is a type of diffusion fabric. It is a sheer white polyester fabric that is designed to soften and diffuse light, creating a more flattering and even light source.
As you mentioned, the amount of diffusion produced by a fabric depends on its density and thickness. In the case of 1/4 Stop Silk, it produces a very slight diffusion of light, which means that it will create slightly harder shadows than heavier diffusion fabrics.
However, 1/4 Stop Silk is still effective in softening and evening out the light source. It can be particularly useful in situations where a more subtle, natural-looking diffusion is desired.
Another benefit of using diffusion fabrics like 1/4 Stop Silk is that they can introduce a significant amount of ambient bounce. This means that the fabric can help to spread light more evenly throughout the scene, reducing harsh shadows and creating a more natural-looking lighting setup.