Regression is re-running previously run tests and checking whether previously fixed faults have re-emerged. New bugs may come out due to new changes in RTL or DUT to unmasking of previously hidden bugs due to new changes. Each time time,when design is changed, regression is done. One more important aspect of regression is testing by generation new vectors. Usually the seed to generate stimulus is the system time. Whenever a regression is done, it will take the current system time and generate new vectors than earlier tested. This way testbench can reach corners of DUT.