air defence code/number

[decimal86]

can anyone pls tell me the origins and more importantly, the use of an Air Defence code/Number?

what would be the likely scenario that i would be required to produce a code that no other aircraft in the sky shares?

[Nightrider]

just one example...when flying in Indian airspace, every flight has to be made known to the Defence Forces, an acknowledgement is the receipt of an ADC in return. This number needs to be noted in the ATC flightplan, which in turn will be rejected if no such number has been obtained yet.