After reading previous page, your're sure to know, what a subtype is. Then you can to skip over this paragraph. For the others, who skipped previous page(click and read it) or cannot remember (click and recall), subtypes are used for specifying only valid range of values for specific situation from any type. For example, if you are measuring humidity and use percentage notation, you probably use only numbers between 0 and 100, because although swimming in the air, there cannot be more than 100% of humidity. On the other hand, you can use only whole numbers, or decimal numbers (the acurately ones will probably do). So the reason of this page is explanation of defining such a subtype.

Subtypes can be defined by several ways.

Once specified, subtype can be used wherever a type is alowed and given range is useful. Thus it can serve as a parent to specify another subtype. Who is now thinking about endles loop of nested subtypes will get a bier from me. Next page - Naming conventions.