A higher protein baking flour that does not contain potassium bromate. The higher protein allows the flour to develop more gluten which results in a more stable baked product and also is desirable for bakers in high altitudes. Potassium bromate is typically added to bread flour to help the elasticity of the baked product. The additive is not permitted for use in the U.K. but it is used in the U.S. Many consider potassium bromate unhealthful and products that contain the additive must be labeled as such in the U.S.
You can purchase umbromated flour at specialty baking shops or buy online at King Arthur, see buy link.