When you log into the dashboard, you will be given two pieces of information: A project key and a bundle set id. You will use these values to submit file size readings, that is, the sizes of a set of files that you want to track.
You can have multiple projects, and each project can have multiple bundle sets. For example, your project may have two separate builds, one for the mobile app and one for the web app. You can use two distinct bundle sets for this.
How to submit
Every time you submit a new reading, the system will pick a previous reading to compare the values with. BundleCop will make this decision based on some additional metadata, like the current branch, or the commit information. It will automatically try to find this information, though in some cases you might need to help it along.