This is G2934Y matte glaze base with opacifiers added. It has been applied to a dark-burning body to demonstrate the comparative degrees of opacity. The stain is Mason 6700 white. While it does not opacify nearly as well as tin or zircon, it does produce a smoother surface.
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.