The scenario is you have an edit process for an account and the primary contact of that account. Step 1 is the account edit, Step 2 is the primary contact for that account. There is no way to define primary contact as the record source. Result from previous step is the account and you can't reference the primary contact or entity reference/lookup on the account.
Would be great is if you could use Previous Step and not just the entity record but a reference on that previous step record.