Tin oxide is most commonly used in glazes where it acts as an opacifier where it is typically added in the range of 5-10%. When used correctly it will produce an opaque, glossy glaze. If used in excess a dull/matt glaze can result. Potters have used tin oxide as an opacifier for hundreds of years.
The addition of larger amounts of tin oxide to lower temperature glazes can increase its refractoriness, i.e. the molten glaze will have an increased viscosity increasing the possibility of pinholing and crawling.
The use of tin oxide is diminishing, with many potters moving towards the use of zircon, which is a cheaper alternative. However, twice as much zircon is required to produce the same degree of opacity when compared to tin oxide.