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when you create a new itinerary you can then drag steps into it in what order you want, there are some orders which doesn't make sense, you should for example not have anything after a send step etc.
[Write a more filling explanation of system assemblies and how order matters]If you want to create new Itineraries there is a prerequisite that the custom itinerary is setup however you will need to setup the step on the itinerary site, ask product team for more information.