This section describes infoset contributions that can be bound to instance data nodes using the bind element. The combination of these contributions to an instance data node is called a model item. Taken together, these contributions are called model item properties.

Model item properties can be distinguished along various axes.

Computed expressions versus fixed properties:

• Fixed properties are static values that the XForms Processor evaluates only once. Such properties consist of literals and are not subject to XPath evaluation.
  
• Computed expressions are XPath expressions that provide a value to the XForms Processor. Such values are recalculated at certain times (e.g. recalculate event, see Processing Model).
  These expressions encode dynamic properties, often constraints, such as the dependency among various data items. Computed expressions are not restricted to examining the value of the instance data node to which they apply. XPath expressions provide the means to traverse the instance data.

Inheritance rules:

Some model item properties define inheritance rules.