If your ultimate goal is to drive Return on Ad Spend (ROAS), it's recommended to start with Gross Revenue Optimization. Optimize for Viewer Retention to maximize livestream traffic.
You may complement lower funnel with upper-funnel goals such as Viewer Retention if you want to also gain more live viewers or acquire new users.
It's recommended only to use Viewer Retention as the optimization goal if Average Concurrent Users (ACU) is a key priority or if ACU is significantly poor.
Learn about Full-Funnel marketing on TikTok.