Excerpt from “Managing your Mobile Data”
What is Cellular Data?
All files, whether a webpage, video, or an ad, have a size. When you access any data that is not saved on your device, it has to be downloaded by an external server. When this happens ideally that file size is the amount of cellular data that will be used. This size will vary based on compressions and locations of the retrieving data. Your data is only used when you do not have a Wi-Fi connection or Wi-Fi Assist is enabled. For instance, if you have a 2KB file it will use more or less 2KB of your cellular data.