Also, keep in mind that hold won't work in some situations with some fare rules. Some rules require immediate ticketing and others have a very small window in which a reservation can be held (less than what book and hold allows). Also, if you are buying an advance purchase ticket and holding the reservation puts you outside the advance purchase window it won't come up as an option.

For instance, if you are trying to hold a 14 day advance purchase fare 14 days out it won't work (since you would be able to purchase the ticket 13 days out).