Hierarchy of molecule, substance, and batch entities in the UCD. Example of the hierarchy is shown for a 2-cyanoacetic acid. Three substances are associated with the molecule: two salts and one neutral substance. Each substance gets a coded letter after the molecule code (e.g., A in UCD01234567-A). Several batches are registered for each substance. Batch codes are generated incrementally upon registration and also serve as submission ID. Batches can be re-assigned to other molecules.