When editing a design, modified constraints are written back to the XDC file that are defined in. Newly created constraints are written to the XDC file identified as the target XDC file when you save the constraints. By default, in a new constraint set, there is no target XDC file. When you create new constraints, you must set a target XDC file when you save the constraints.
To indicate that constraints need to be saved, the Save Constraints toolbar button is enabled 
. When you click the Save Constraints toolbar button, the Save Constraints File dialog box lets you choose an existing XDC file in the active constraint set, or create a new file to add to the active constraint set.
If an XDC file is set as a target, the word "(target)" appears next to the file name in the Sources window. You can change the target XDC file at any time using the Set as Target Constraint File right-click menu command in the Sources window.
