When pulling the torque converter and changing the transmission front seal there is a little round thin metal clip that's a little bit bigger than the seal is that necessary when putting it back together is it a lock clip or what and does it matter if it was left out when reinstalling please let me know thank you in a situation thank you

 

Best to reinstall it after a seal change, kind of an insurance policy as the labor required to change out a $12 seal can get expensive. But can be left off. It is a retainer to help hold the seal from pushing out. Had problems with really early units pushing out the seal. Later models have the drain back hole enlarged helping to keep fluid from collecting behind it and forcing it out of place.
Brad