Thal minor is often also referred to as thal trait. It means that only one of the two beta hemoglobin genes carry the thal trait. Some intermedias also carry just one thal gene, but their condition is severe enough to be classified intermedia. Recently, trait has sometimes been referred to as a minor with few if any symptoms, whereas if there are significant symptoms, it is called thal minor.
This is a good demonstration of the vagueness of thal classifications. The classifications were originally based on the amount and severity of symptoms, but with the DNA testing available today, these classifications have been modified to some extent, leading to even more confusion.