Altium Designer Documentation
...
Dialog Reference
A to F
Choose Document to Reference
Created: July 28, 2015 | Updated: June 16, 2017
| Applies to versions: 15.1, 16.0, 16.1, 17.0 and 17.1
Now reading version 17.0. For the latest, read: Choose Document to Reference for version 21
The Choose Document to Reference dialog.
Summary
This dialog allows the designer to choose the target schematic document that is to be referenced by the sheet symbol whose properties are currently being defined.
Access
The dialog is accessed from within the Sheet Symbol dialog, by clicking the 
button associated to the Filename field.
Options/Controls
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 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.