The Z-report is the finalized X-report and contains all sales data of a checkout day. This means that the data in the Z-report can no longer be changed, not even retroactively, neither by you nor by us.
When you tap on "Start day", all transactions are stored in an intermediate report, the X-report. As soon as you close the day in your MINI by orderbird, the transactions are finalized and saved in a Z-report.
You can open all Z-reports via [Main menu] > [Reports] and print them as often as you want. The Z-reports are tax compliant.