The system that deals with users who request deletion in order to evade restrictions that are placed on their accounts at the time of deletion.
As explained in the May 2014 moderator newsletter:
Anti-Recidivism System Added
[...] We log data on such accounts at time of deletion, which is then referenced should a new account from the same individual be created. If the user was suspended at the time of deletion, then the new account will inherit the remainder of the original suspension. If the user was blocked from posting questions at the time of deletion, then the recreated account will be restricted to posting only one question per week until the quality of their contributions is demonstrated. [...]