This is the name of your VLF-ONE reusable part that will pop up when the user mouses over an instance list entry of this business object when the Framework is running as a VLF-ONE application.
The reusable part must have as its ancestor VF_AC021O.
In order for the popup to be shown, the business object that displays the instance list must have Enable Pop Up Panels checked, and the end-user must not have disabled instance list popups. The popup panel can be any size you want.
This property is in the Business Object Instance List tab.