The app is the interface between your service and the Vertopal API. This allows integrate your service with Vertopal to use its services, so this was created by a developer and maybe isn't owned by Vertopal.
Suppose, Your service wants to convert a file. This service provides the input file with the credentials and data request to Vertopal through the app. Vertopal runs the convert task and after it, provides the converted file to your service through the app.