Credits are directly related to the cloud compute time needed to render a scene. The cost for upload, download and cloud storage of the scene is not charged. The compute time is displayed as the 'render time' parameter when reviewing a job in Chaos Cloud. It includes starting up the software on the render machine, starting up V-Ray, loading the scene, pre and post scene-rendering steps, like global illumination calculation, geometry compilation, denoising etc., actual rendering of the scene, generating and making the output images available for download.
It's important to mention that the last step regarding the output images (which may include the render elements as well, if there are any) may take some time, even though the low-resolution preview images are ready and the rendering of the job is completed.