I would suggest that you read up on Project Customization. BG is indeed for Bug and User indicates that it is a user defined filed, that didn't come from a template, and 03 is the third custom Bug field they have created in this project.
In ALM in a project that you are an administrator of click on Tools | Customize and then click on Project Entities to see all of the areas system, template custom, and project custom fields that you can access in your workflow code.
Craig
