An example of a digital decal that has been fired at 1500F onto a matte cone 6 glaze. Notice the glossy, square around the graphic. This is where the decal paper was cut prior to transfer. This glossy layer appears by design, the decal paper has a thin layer of glaze and your inkjet design is printed, reverse reading, onto that. After transfer and firing that glaze thus insulates the color from coming into contact with anything that might leach it. It is thus wise to trim carefully around designs such that the glossy pattern does not detract from the appearance (as it does here). You can get decals without the glaze covering, but may not melt enough to be durable on the glaze surface.
This process is done by printing a design (using ceramic inks) onto a film coated decal paper, drying it, then transferring the film to the fired ware. But beware of problems.