Singlebox and WebCatalog are separate apps, but they share the same account and subscription system.
Using a WebCatalog account means you don't need to create and manage a separate account for Singlebox. If you already use WebCatalog, you can simply sign in to Singlebox with the same account.
Your WebCatalog subscription also works with Singlebox, so you don't need to purchase a separate subscription to unlock paid features.
You do not need to install or use the WebCatalog desktop app to use Singlebox. Singlebox works independently and can be downloaded from singlebox.app.