Through the alternate track and trace, you are able to trace the air waybill number of one airline, in the system of another airline.
For example, if you wanted to track air waybill number 125-12345675 (which is the British Airways air waybill number) in the Air France system, then you would use this facility. 

It is not necessary to use the alternate track and trace where you are searching for an air waybill number of an airline in the system of the same airline. In such instances (which are most instances) all you need to do, is to key in the prefix of the airline and the air waybill and it will automatically search in that airline. Only use the “alternate airline search” facility if you whish to search the air waybill of one airline in the system of another airline.