A property descriptor holds the type information for a typed data element. For example, a property descriptor for a data element of type Money would refer to the single instance of the type called Money. The typed data element identifies its property descriptor using its descriptor attribute. A non-typed data element has a null value in this attribute.

Property descriptors use parameters to describe their type. These parameters take the form of key-value pairs that can add attributes to the type, set attributes as mandatory, or specify an initial value for an attribute. For example, an Account instance must have an account number. The descriptor for the account number field uses the isMandatory="true" key-value pair to indicate this. The property descriptor and the data element store their parameters in a Hashtable. For the typed data elements, if the shareDataDescriptors flag in the btt.xml is set to true, the toolkit passes the data element's Hashtable of parameters in each call to the DataElement descriptor so that the default descriptor and its set of converters and validators can use the appropriate parameters instead of the default ones contained in the property descriptor's Hashtable to perform their tasks.