A constraint set is one or more constraint files that are maintained independently and concatenated into the in-memory design for analysis and implementation. A constraint set defines the constraint files to be used at specific moments, or under specific conditions, in the design process. By defining multiple constraints sets, you can, for example, specify different active constraints to resolve floorplanning and timing problems.
The XDC files can be used during synthesis, implementation, or both. By default all XDC files are set to be used in both synthesis and implementation, as defined by the USED_IN property on the constraint file. To change the USED_IN property, select the XDC file in the Sources window, and check or uncheck the appropriate box in the General view of the Source File Properties window. Refer to the USED_IN property in Vivado Design Suite Properties Reference Guide (UG912) for more information.