Externally defined models with src attribute
For efficiency, it can help to divide up the user interface of a larger form into separate files that share the same model.
A src attribute on model can be used to ensure the same model is used for all the forms.
Note that the resource attribute should also be added, with the same semantics as is done for the instance element. This is needed for document-based applications of XForms, such as ODF, in which the initial document is saved and reloaded. Such documents would use a resource attribute, not a source attribute, so that the document could obtain its initial model from a central location, but once the document is instantiated for use by an end-user, it has its own copy of the model to be used thereafter. This is necessary because the model contains the instance data, so if src were used (on model or instance) then the instance data collected before save would be lost when the document is reloaded.
It may be that this issue is subsumed under ModelEnhancementRequirements and takes a different direction.