A threshold list is
a list of securities whose transactions failed to settle for five consecutive settlement days.
Threshold lists are published by various exchanges by SEC regulations.
Settlement failures may be indicative of improper naked short selling.

A broker-dealer with fail-to-deliver positions for 13 consecutive settlement days must immediately close out the fail to deliver position by purchasing shares in the open market. Trading Insights: There are several ways to view the Threshold List. Some investors consider these stocks as possible short squeezes. 

NYSE Threshold List