When working in Tableau Prep, we usually insert a new flow step after finishing the last one. For example, we add a dataset and then add a clean, aggregate, or join step. It looks something like this:
However, when working with more complex flows, we often have to insert another step between already existing workflow steps. In this case, you have 2 options where to insert your new workflow step. When you hover over the line between your existing steps, you see that a plus sign appears at 2 different locations: at the beginning, and somewhere in the middle (or end).
1. Insert at the beginning
Clicking on the first plus will insert a new separate flow from your first dataset. It creates a new branch like in the following picture.
You would want to use this step if you do not want to continue the previous cleaning flow (in my example: I do not want the step that goes from People to Clean 1), but would rather like to create a complete new step. But be aware that any changes made in the Clean 1 step will NOT transfer to the clean step that you just created, as these are two different branches.
2. Insert in the middle
When you choose the plus sign in the middle, Tableau Prep will insert the new step between People and Clean 1.
In my example you can see that Clean 2 was inserted between People and Clean 1. You may use this if you forgot a cleaning step and want to add it before your next step. Of course the same counts for any other workflow step such as join or aggregate. But be careful, if you insert a clean step like I did above, any changes you made in Clean 1, will not show up in Clean 2 yet, since those changes have not occurred yet. So keep the chronological order in mind.
Hopefully, this clarifies which of the plus signs to choose should you want to insert a new flow step into an existing workflow.
Illustrations created by Lisa Hitch (with screenshots from Tableau Prep)