A Calcutta is an option, usually part of the handicap championship event options.
Shooters who wish to participate in the option pay an entry fee. Those shooters are then sold at an auction, either singly or in pairs (ala the slaves being sold at auction in old Calcutta, India).
The money obtained from the auction is added to the entry fees for the option.
Calcuttas are often paid 40/30/20/10 percentage, with the buyer getting 70 percent and the shooter getting 30 percent. Although other payoffs are possible.
When sold in pairs, each shooter's score stands alone, it is not added to the score of the other shooter in the pair. Usually long yardage shooters are paired with short yardage shooters, although there is no "rule" that requires that. It just makes it easier to sell short yardage shooters when there are a large number of shooters participating.
Calcuttas can get large. Calcuttas with $20,000 to $30,000 (total) in them are not unheard of, even today.
Calcuttas are everywhere, not just big shoots, not just trapshooting. A lot of small clubs do it on a regular basis and they are stand alone. Also beware that many states consider it an illegal form of gambling.
That said I am going to one on Sunday the 9th, advertised on here.