Taken from http://www.airport-int.com/categorie...ng-systems.asp :

"The Industry standard Five Level Screening Process is based on the following: -

Level 1: Automated evaluation of the X-ray image by the X-ray Machine.

Level 2: Operator analysis of the Level 1 image at a workstation(s), carried out whilst the bag continues in transit.

Level 3: A more in-depth analysis of the original Level 1 image at a separate workstation(s) or subjecting the bag to a separate X-ray process using a different X-ray technology, for example Computer Tomography (CT Scanner).

Level 4: Re-uniting the passenger and bag and carrying out a manual search.

Level 5: In the event that the passenger cannot be found, then the bag is considered a bomb threat and dealt with accordingly."