Elementary POS allows you to have multiple cash registers at the same time. The app can also issue receipts offline. That is why each of your cash registers has a different prefix in the receipt number. This ensures that there are no two receipts with an identical number. And to prevent gaps in numbering, it is not allowed to delete receipts except for the last one created.
If you need to cancel a receipt other than the last one (for example when issuing a refund), you have to invalidate it by issuing a new, negative receipt with the same value. This brings the sum of sales to zero.
The receipt numbering can be turned off in the settings. After that, the deletion of receipts is not limited.
