Whether you use one or several XDC files for your design, organize your constraints in the following sequence.
## Timing Assertions Section # Primary clocks
# Virtual clocks
# Generated clocks # Clock Groups
# Bus Skew constraints
# Input and output delay constraints
## Timing Exceptions Section # False Paths
# Max Delay / Min Delay # Multicycle Paths
# Case Analysis # Disable Timing
## Physical Constraints Section
# located anywhere in the file, preferably before or after the timing constraints # or stored in a separate constraint file
Start with the clock definitions. The clocks must be created before they can be used by any subsequent constraints. Any reference to a clock before it has been declared results in an error and the corresponding constraint is ignored. This is true within an individual constraint file, as well as across all the XDC files (or Tcl scripts) in your design.
The order of the constraint files matters. You must be sure that the constraints in each file do not rely on the constraints of another file. If this is the case, you must read the file that contains the constraint dependencies last. If two constraint files have interdependencies, you must either merge them manually into one file that contains the proper sequence, or divide the files into several separate files and order them correctly.