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.
Example
HTML
<div itemscope id="amanda" itemref="a b"></div>
<p id="a">Name: <span itemprop="name">Amanda</span> </p>
<div id="b" itemprop="band" itemscope itemref="c"></div>
<div id="c">
   <p>Band: <span itemprop="name">Jazz Band</span> </p>
   <p>Size: <span itemprop="size">12</span> players</p>
</div>