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In some cases, you will find yourself in a situation where you want to use an external accounting system, such as DATEV. These systems use their own reference numbers to identify advertisers. If you want, you can put these numbers in the field at the bottom of the page. If you do so, it will appear on any financial document you create for this advertiser via the platform.
Don't forget to click the 'save' button at the bottom of the page when you are done.