By ‘Save Changes’ button, I think Ihor was meaning the regular Submit button at the bottom of the page.
If the listing form contains something preventing it from proceeding (e.g. missing some input on required fields), I believe the Submit button acts like a Save button and stores the progress as a draft.
The Save Changes button appears when you’re editing an existing listing. This means the button will show up after you’ve initially submitted a listing and are making changes to it.
Sorry for the confusion.
Currently, there’s no option to save a listing draft during submission. The Save Changes button only works after the listing is added, but some details [like images and category] remain when you return to the Add Listing form.