The Keep Existing Constraints check box is designed to support an additional “merge or replace” functionality when you import SDC files.
Select Keep existing timing constraints to preserve all existing timing constraints that you have made using the Timer GUI or previously imported file. If you import a SDC file and you have this check box selected, Designer merges the existing constraints and the constraints existing in the SDC file. In case of a conflict, the new constraint has priority over the existing constraint.
The Keep Existing Constraints option is On by default. With this option On, your timing constraints from the imported SDC files are merged with the existing constraints. When this option is Off, all the existing timing constraints are replaced by the constraints in the newly imported SDC files.