What does your "recommended firing range" mean?

The "recommended firing range" is the temperature range over which we expect the clay to have a good glaze fit, with no crazing or peeling. Earthenware clays are designed to be used with earthenware glazes, and stoneware with stoneware glazes. Trying to use earthenware glazes on a stoneware clay is likely to give poor results. Where clays are shown with a wide firing range, for example 1110 to 1290C, they can be used with earthenware glazes at the lower end of the range and stoneware glazes at the upper end.