A project is a single configuration of your custom bag. It includes details such as bag style, size, material, options, etc. You are allowed to upload as many artwork files to the project as you want. When you upload an artwork file to a project, it will be based upon the project's configuration details.
If you have multiple artwork files and they all have the same configuration, it is best to upload all of the artwork files to the same project. There is no limit to the number of artwork files you can add to a single order.
Why is this important? Eventually, you will be able to create and submit production orders from your project. If all of your artwork files are in the same project, you will receive better per-unit pricing when placing an order.