It is common for attributes to contain the same type of information. For example, all temperatures in a system are likely to share units, high alarm level, and value. To avoid repeating this information for each and every temperature in the system, we use a type. A type is the prototype, or class, of an attribute. You define a type and its properties first, and then define attributes of that type on assemblies. When the assembly is instantiated, its attributes are instantiated by creating an attribute and then assigning the properties to it that are associated with the attribute's type.

There is an alternative to using types and attributes as described here, a private attribute. A private attribute provides a one-command (private_attribute) means of creating an attribute on an assembly without having to define a type. This prevents the attribute properties from being shared across more than one attribute in the assembly or in other assemblies, but is easier to use when defining simple hierarchies. (See Example 2.)