Saving progress in an online form prior to submission (need to save the data within the form)

Hi there.  Which version of Nintex Forms are you using?  Are you using the Responsive Designer or New Reponsive Designer?

Thanks for the info.  Which version of Nintex are you using for your form -- SharePoint on-premise, SharePoint O365, or Nintex Workflow Cloud?

In the Responsive Designer and Classic Designer in O365, buttons have options for "Save", "Save and Continue", and "Save and Submit".  Using these options would be a good route to go.  Since you're using the New Responsive Designer, unfortunately those button options don't currently exist.

It's not for New Responsive Designer, but you could build something along the lines of this solution: Save as Draft button.  Add a "Submit form" checkbox control that users check and click the Submit button when they're finished and ready to submit the form.  If there are not ready to submit the form, leave the box unchecked and they can click Submit to "save" the form. They can then come back to it later to finish.  The "Submit form" checkbox could save to a list column and you could have a workflow only execute when it is checked upon submission.

Here's a list of current New Responsive Designer limitations in O365: New Responsive (Office365) - Current limitations

Also, be sure to check the Nintex User Voice site to provide feedback on this and other desired product features: Nintex User Voice 

If you're using SharePoint for O365 and the form is a list form, you could save the "draft" form to a list. The workflow could kick off when the item is created, then wait for the "Submit form" checkbox to be checked to proceed and execute the rest of the business logic. you could use the "Wait for Field Change in Current Item" action to accomplish this. If a user fills out a new form completely on the first try and checks the box, then the workflow will execute without waiting on this action. You could also have the first step of the workflow send the user a link to the form so they can open it back up later if they need to go back and complete it.