Just to dive a little deeper. This is from the FDA's website:
7. What does gluten cross-contact mean in the context of the final rule?
The term “cross-contact” is not defined in the final rule, but gluten cross-contact generally refers to the unavoidable presence of gluten in a food due to contact with a gluten-containing food (wheat, rye, barley, or their crossbred hybrids like triticale). The U.S. Grain Standards allow commodity grains, legumes, and seeds to contain a small percentage of another commodity grain as a result of commingling during production. The presence of gluten in a grain, legume, or seed that is naturally free of gluten from this type of commingling is an example of cross-contact. The typical practice of crop rotation, as well as the shared use of harvesting and transport equipment and storage silos, often results in gluten-containing grains coming into contact with other grains, legumes, and seeds that are naturally free of gluten. Furthermore, manufacturing facilities that use shared production equipment to produce foods both with and without gluten could result in gluten cross-contact. **Any grain other than the gluten-containing grains** of wheat, rye, barley, or their crossbred hybrids like triticale **can be labeled gluten-free, if it meets the definition, including that the presence of any unavoidable gluten due to cross-contact situations is less than 20 ppm.**