This dialog allows the designer to choose the target schematic document that is to be referenced by a newly-created sheet symbol, when using the Create Sheet Symbol From Sheet or HDL command.
The dialog is accessed from the Schematic Editor in the following ways:
The main area of the dialog lists all candidate schematic documents for the project, that can validly be used as the target reference sub-sheet for the new sheet symbol. Each document is listed in terms of its name, and path to the folder in which it resides.
Simply click on an entry to select the required document.