Here is an example showing how the selection of the size of a banner and the selection of a zone fit together. An advertiser creates a banner to fit one of the recommended IAB size formats. A publisher creates a zone of the appropriate size, and then dictates the delivery position via the use of invocation tags in their website.
There is no need for any communication between each advertiser campaign and each publisher website, the adserver takes care of all that complexity, leaving you to concentrate on your designs. It’s easy to see why it’s important that the IAB preferred sizes mesh well together.