Likelihood of Success of the Act
The potential for causing the event by defeating the countermeasures. L2 is an estimate that the security countermeasures will thwart or withstand the attempted attack or if the attack will circumvent or exceed the existing security measures. This measure represents a surrogate for the conditional probability of success of the event. (Conditional probability of success of the event is the measure of vulnerability (V), so therefore L2 and V are synonymous: L2 = V.).
Source:API STANDARD 780, Security Risk Assessment Methodology for the Petroleum and Petrochemical Industries, First Edition, May 2013. Global Standards