How pallet changer works?

I want to know the concept of pallet changer codes i.e how to toggle the reference position for different pallets and how machine/program knows which pallet in action?

Any reference configuration and job program may be of great help.

Thanks in advance.

in the simplest case as with the toolchanger, tracking via variables and offset-tables.

i think it's also possible via RFID's or Barcodes with external code

Well, there is another idea that might help in bypassing the mechanical arrangements of a pallet changer.

If a larger table is used with moving gantry mill type machine, we may use front half of the table for one job and rear half for another.

But how to offset the job zero from one half to another half?? Will G54/G55 work in this case?