How does Submit Interaction Button Work?

The "Submit interaction" trigger is reserved for graded, survey, and freeform question slides. Storyline has built-in functionality for handling those types of slides. By default, the "Submit interaction" trigger is added to the Submit button. But that trigger could be added to other objects as needed.  For example, you could use a built-in Next button or a custom button to submit the question interaction.

However, Storyline can only "submit" its own interactions: the ones on graded, survey, and freeform question slides. It can't see that you've programmed your own interaction.  So even if you add the Submit button to another type of slide, you won't be able to assign the "Submit interaction" trigger. 

I hope that clarifies it for you!