This dialog allows you 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 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 legitimately can 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. Click on an entry to select the required document.