When your customer has changed one or more objects in the database you get a warning when you import a transport.
Opening the Object Compare Sheet gives you an overview of the changes the customer has made since the last transport. In the following screenshot you see that the customer has changed the customer table.
It is possible to analyze the changes with your compare tool or confirm the changes with the confirm button. All modifications have to be confirmed before you can import a transport.
Set options "Save C/AL Before Import" and "Save C/AL After Import" on "If changed" or Yes.