<xh:head>element. An XBL component does not have an
<xh:head>element, so Orbeon Forms supports placing models in two places:
<xbl:implementation>element. In this case, models are identical for all instances of a particular component.
<xbl:template>element. In this case, models can be different depending on the component instance, since the XBL template can parametrize its elements and attributes.
<xbl:implementation>is to place new methods and properties. Orbeon Forms uses XForms as the implementation or "scripting" language of XBL components, so it does not have methods and properties. But models, through events, can implement behavior, so allowing model placement under
<xbl:implementation>seems to fit the intent of XBL.
rebuild(if document structure changed): update binds structure/dependencies
recalculate(if value changed): perform MIPs and calculated values updates
revalidate(if value changed): perform instance revalidation
refresh: updates the UI and dispatch UI events