This dialog allows you to choose the target schematic document that is to be referenced by the sheet symbol whose properties are currently being defined.
The dialog is accessed from the Properties panel in Sheet Symbol mode by clicking ... to the right of the File Name field in the Source region.
The main area of the dialog lists all candidate schematic documents for the project that can be used as the target reference sub-sheet for the sheet symbol. Each valid document's name and path to the folder in which it resides is listed.
Click on an entry to select the required document.