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You will receive a confirmation email upon booking and and on your voucher you can read how to reserve your time slot. Although you can do this at the Colosseum Ticket Office, it is sensible to do this online in advance because otherwise there is no way of securing your preferred date and time.
The one I recommend you to select is the one including all areas. In 2024 these are called 'Full Experience tickets'. They include the underground area of the Colosseum, the arena floor, the second floor with exhibition and access to the Forum.