Calibrated paving stones are mechanically processed to be the same thickness. They are slightly more expensive than uncalibrated ones, but they’re also much easier and cheaper to lay as they require less levelling out. They can also be smoother than uncalibrated slabs as the process requires a selection of the flatter slabs.
Calibrated stones will have a more uniform surface and even depth measurement — while uncalibrated stones will still have their natural and less precise variations.