This is concrete sand. It is usually used for making concrete, but is also perfect for using as part of the cob mix. It usually has particle large and small and all the particles are angular. These qualities makes for he strongest cob.
This sand is often used for stucco cement plasters on the outside of conventional houses. In earthen construction it can work well for cob, earthen floors and brown coats in the form of a sand/clay-soil plaster. Just like concrete sand, it has different size grains and all the grains are angular. The particles are just a bit smaller so it can work better as a plaster.
This sand is finely screened, but the grains are still angular. Because it is so light in color, it is ideal for finish earthen plasters when you want the end products to be light in color. You can also buy cheaper sands, sometimes sold as sand box sand(or play sand) that is darker. Or, you can screen either of the above sands through a window screen and then use that for a finish plaster.