ICT is a complex testing machine involving electronic+pneumatic component.
THe default value is stored by user for each PCB board to be tested. ICT will compare this vlaue the reading that it gets when ICT is functioning.
THe main idea is using "Bed of NAils" approach. the pin of nails will touch the PCB on surtance point(that why every PCB designed have a test point node).
The pin is supplied with a little voltage making the component tested saperately.
The test point on PCB is eleborate in the program thus user can see the fault pin no using GUI and thus knowing which is the broken component,short cct or open cct that occur.