A regression system is a set of testcases that are run regularly (sometimes
several times a day) to make sure that nothing in the SOC is broken since the
previous runs. Depending on the complexity the number of tests could be very large
(more than a million tests are not unheard of). These tests are typically
constrained random ones, and are also self-checking.
(ie, they output PASSS/FAIL). j
At the end of a regression run, you would know the list of “new” failures, failures
that did not exist in the previous run.
Each company/team has their own regression processes in place.