As an example of data when you open a web page on your iPhone data is transmitted to the host site requesting the page and data is received from the site containing the information and graphics on the page. How much data is involved is a function of the page itself, the graphics on the page, and in the end the skill of the site developer. There really is no way of knowing in advance. Think of data as anything used by the browser or for that matter any app on the iPhone that needs to access data from the internet. There are so many variables that any effort to predict the size of the data transfer is doomed to failure.
