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Model Item Properties

An individual property that can be applied to a node is called a model item property. Some of the properties are XPath expressions (called computed expressions in the specification), which the XForms Processor tracks and reevaluates as necessary:

The remaining properties are static, and don't get reevaluated:

Some of the model item properties have an effect on child nodes as well. The rules for this behavior can be summarized like this:

required="true"
required="true(  )"