Passthrough properties

The term element is used as a shorthand for XML element. It is sometimes useful to distinguish between leaf node and non-leaf node elements, which in PDS4 terms are referred to as attributes and classes, respectively. The term attribute is therefore used as a shorthand for PDS4 attribute; XML attribute is always spelled out.

The PT template language is based around assigning properties to elements of a template. These properties consist of a single keyword - expression pair and are declared on elements in the template via XML attributes under the PT namespace, i.e. pt:keyword="expression". The below table summarises the key characteristics of the properties that the language currently defines. Their individual usage patterns are described further in the next section.

Keyword	Expression type	Evaluated type	Default	Inherited
sources	string (key)	Sources (one or more labels)	None	yes
fetch	XPath	boolean	False	yes
multi	XPath	boolean or integer	False	no
required	XPath	boolean	True	yes*
fill	XPath	string**	None	no
defer	XPath	boolean	False	no

* terms and conditions apply; further details below. ** fill expressions do not have to result in a string directly; see the relevant section below.

Initially an element will be assigned the default value of each of the PT properties. Unless a property is explicitly declared on a given element via an XML attribute, for some it will be inherited from the element's parent. Certain properties are only applicable to leaf nodes (attributes), while others modify the evaluation of another property.

During pre-processing of a template, the effective values of the PT properties of each element will be derived. Together these constitute an element's state, which the pre-processor will check for consistency.