Hi Fabian99,
Welcome to Community! Intuit Interchange Format files are a great way to import and export lists and transactions to and from QuickBooks Desktop. When you encounter an error, you'll have the option to review and resolve them in the .IIF file. Click here to review .IIF sample files and headers. Please be advised that Intuit doesn't offer technical support for .IIF files. If your file came from a third-party app, you'll need to contact the app’s technical support for assistance.
If you have any other questions, please feel free to reach back out. We'd be happy to help!