A data holder is an object that has one of the following types:
XML elements and attributes are typically used for permanent storage. A Parser, for example, stores its output in data holders of these types.
Variables are used for temporary storage. For example, a Parser can store data that it extracts from a source document in a variable. It can process the data further before creating the output.
Every data holder has a data type. In the case of elements and attributes, the data holders are defined in an XML schema that you must supply. Variables are defined in an internal schema, which you can customize by adding user-defined variables.