Tier 3: A challenge to the barrier system that progressed along the path to harm but is stopped short of a Tier 1 or Tier 2 PSE consequence. Indicators at this level provide an additional opportunity to identify and correct weaknesses within the barrier system.

Tier 4: indicators represent performance of individual components of the Operating Discipline and Management System.
Indicators at this level provide an opportunity to identify and correct isolated system weaknesses. These indicators are indicative of PS system weaknesses that may contribute to future Tier 1/Tier 2 PSEs.