Object keys based on numbers reference every message sent under that step index including A/B tests and manually sent emails.

Object keys based on IDs reference the exact email copy that the email sent with. This is especially useful with A/B tests. To get the IDs of each step, pull the campaign and look for the steps and other_steps parameters.

Campaign Step Stats supply stats for one message. For example, if a contact replies to the second step, it does not retroactively apply to the first step.