Answer
A selective sync applies only specific resources within an Application rather than all of them. It's useful for large apps to push a single changed resource quickly, or to work around a problematic resource. In the UI or CLI you choose which resources to sync; note it can leave the app partially applied.
argocd app sync my-app --resource apps:Deployment:webReal-world example During an incident, an engineer selectively syncs just the fixed Deployment instead of the whole large application.