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:
This almost certainly doesn't have the intended effect of making the node always required. Computed model item properties are XPath expressions, and under XPath rules, "true" is interpreted as a node-set matching all element nodes named true. Unless there is such an element, the expression will evaluate to an empty node-set, which is interpreted as an XPath boolean false! Always use the functions true( ) and false( ), which return the expected XPath boolean values.
It's not wise to attach an editable form control to a calculated node, since the XForms specification gives recalculation engines the freedom to recalculate at nearly any given time. As part of the recalculation processing, any new information entered into the form control will get wiped out.