You can use internal codes for anything you desire but usually they are used for coding and reporting on internal budgets or departments.
Internal codes can be added individual fundraiser records (and donate button post records). You can find and add new Internal Codes by going to Account Settings > Internal Code
Unlike labels each Internal code has its own column in both the Fundraiser Export to CSV file and also in the Fundraiser Type report in Report Builder.
You can add multiple internal codes to each fundraiser (or donate button post). To do this simply go to the fundraiser record in GivePanel and you will see a field where you can enter multiple internal codes.