The RCP product export story is based on a .product file. This file contains all the data necessary to build a plugin-based or a feature-based product. Note that this file is a development-time artefact used by PDE only. It is NOT read or interpreted by the runtime and must/should not be included in the product being built.
To create a product configuration, select File > New... > File > Product Configuration.
The only restriction is that the file extension be .product. Otherwise, the product configuration could be place in any folder, in any project in the workspace.
To initialize the configuration, you have three options:
In our RCP mail example, we already have a product defined: com.example.xyz.product. So the second option is best. Select it and press Finish.