Tcl scripts can be added to project constraint sets like any regular XDC file, except that the
XDC files are managed by the tool, and not Tcl scripts. Any constraint defined by a Tcl
script and edited by the tool cannot be saved back to the Tcl script automatically. If
you need to save your edits, you must export all the constraints in memory to a file and
use this file to update your script manually. When opening a design in memory
(open_run), the Tcl scripts are sourced after the XDC files. In a
Non-Project flow, this is equivalent to explicitly sourcing the Tcl scripts after
loading the XDC files with read_xdc. For more information on using XDC
files and Tcl scripts in a constraints set, see the
Vivado Design Suite User Guide: Using
Constraints (UG903).