What are the units of the number returned by the Cartesian3.distance function?
The short answer is meters. Cesium almost always represents data in SI Units, meters, radians, seconds, etc… If there’s a reason for the data to be in another unit, we try to make that clear in either the function/variable name or doc.
A slightly longer explanation is that Cartesian itself doesn’t care and is unitless. If you create two cartesian instances using kilometer measurements for example, the results will be in kilometers. If you mix units, the results will simply be incorrect. No internal checking is performed, but other Cesium functions expect it to use meters.