An XFDL scope identifier, or sid, uniquely identifies an element within the scope of its logical parent. Each <page> element must have a sid attribute that uniquely identifies the page within the surrounding XFDL form element. An item element must have a sid attribute that uniquely identifies the item within the surrounding page element.
When an item appears in an <xforms:group>, <xforms:repeat> or a case of an <xforms:switch>, then the scope identifier uniquely identifies the item within the group, switch case or repeat template. When a repeat template is instantiated for each node in the repeat nodeset, the items generated are unique within each template instance (each logical row of a table).
In XFDL, each option element is defined to be uniquely identified within the scope of the surrounding item element by its XML tag, which is why options (and array elements) do not require a sid attribute.