Genetic defects in insulin action. Again, several disorders have been identified.
Diseases of the exocrine pancreas. If the pancreas is damaged or destroyed, such as by cancer, injury, or infection, the beta cells of the pancreas that make insulin may be damaged or destroyed.
Endocrinopathies. If there is a disorder causing excessive secretion of hormones such as thyroid or growth hormone, diabetes may develop. Typically, the diabetes resolves when the other hormone problem is fixed.
Drug- or chemical-induced diabetes. The most common prescription medication that can cause diabetes is the class of drugs called steroids.
Infections. Some viruses have been associated with beta-cell destruction and subsequent diabetes.
Uncommon forms of immune-mediated diabetes. Other autoimmune disorders may be associated with diabetes, including the "stiff-man syndrome" and lupus (SLE).
Other genetic syndromes sometimes associated with diabetes. People with other chromosomal disorders, including Down's syndrome, Turner's syndrome, and Kleinfelter's syndrome, may have diabetes.
One might imagine that the folks who dreamed up this official classification might have continued beyond the two "types", so that (for instance) they might have called GDM by the label "type 3 diabetes." But they didn't; there are only two "types" and a whole bunch of other disorders that didn't get a number of their own.
In my opinion, anyone using unofficial terminology, such as type 1.5 or type 3, should acknowledge that it's unofficial, and clearly define what they are talking about. Otherwise, the confusion will continue.