There are a number of common contests that exchange QSO numbers. This would be simply a numeric that is entered by the use. In contests, the exchange might include “you are my 24th contact” . Having a log book produce a sequence number would also be good, but it sort of does anyway with the QSO number. Where it should go in ADIF or Cabrillo is not something I know offhand, but it would be worthy of understanding. Collecting it up front probably is a simple change, however.