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In the changes history window Leon shows chronological list of all changes (the latest on top). The colours indication shows added data in green, changed data in yellow and removed data in red.
When you click on the changed detail, Leon highlights previous entrances of the same detail. On the screenshot below you can see highlighted STA 15:55, then also 15:43 (previous STA) and 14:42 (initial STA).
If the flight has been deleted in Leon you can still check its history (as long as the option to keep deleted flights as crossed-out is enabled) in case you want to know who deleted it and when.