The global attributeitemref properties, which are not descendants of an element with the itemscope attribute, can be associated with the item using an itemref.itemref provides a list of element id's (not itemids), with additional properties elsewhere in the document.
The itemref attribute can only be specified on elements that have an itemscope attribute specified.
Note: the itemref attribute is not part of the microdata data model. It is merely a syntactic construct to aid authors in adding annotations to pages where the data to be annotated does not follow a convenient tree structure. For example, it allows authors to mark up data in a table so that each column defines a separate item while keeping the properties in the cells.