A Poly Item can be a property data source of another item (such as the Deliverable property of a Project Activity2) or a Related Item.  When selecting existing Items, the Search Browser dialog will display an additional pull down list of the Poly Sources.  You can search across all the Poly Source Items at once as shown below.

 

Or, you can select a specific Poly Source from the pull down list to confine the search to a single type. In the example below, Parts was chosen as one of the possible Deliverables.