Properties that are not descendants of an element with the itemscope attribute can be associated with an item using the global attribute itemref.
itemref provides a list of element IDs (not itemids) elsewhere in the document, with additional properties
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.