How to track the completion of a CTA
In your call-to-action, as soon as someone engages with a button or form that has a "Track completion of CTA" action added, they will have "completed" this CTA.
This action exists and is automatically applied to buttons and forms because this action contributes to the "Completion Rate" stats of your CTA.
For example, the first step of your CTA could have a button that displays the second step, which has a form. You may not consider the CTA "completed" at this point, so you could remove the "Track completion of CTA" action from the first step's button.
And with the "Track completion of CTA" action added to the 2nd step's form, your "Completion Rate" stats will be for those who complete both of your CTA's steps.
Also, if your CTA is being added to a goal in a Flow, the "Track completion of CTA" action is what determines when to run the actions you've added to that goal run.
Because you can control which button or form in your CTA has the "Track completion of CTA" action, this allows you to control when to trigger the actions of any Flow goals the CTA may belong to.