According to the masterclass guidelines, the case instance is created when the STP fails.
The STP fail would trigger the case creation which will be assigned to an underwritter.
With that being said, since the case is only created when the STP fails so you have to:
1. Integrate REELS on your SSD flow first before creating a case.
2. Depending on the REELS output create a case(which will be a human task assigned to underwriter) or generate the policy.
Please check the ALPHA masterclass guideline to get more clarity on how the application should flow, also check the attached JBPM image there. I have attached the link alpha masterclass link below.
Neutrinos Alpha Master Class Assignment: Building a New Business Policy Underwriting System on the Alpha Platform - Community / Training, Academy & Certifications - Neutrinos