Vertical Accuracy Definition

The meaning of the vertical accuracy is defined clearly in the documentation. It adds a +/- value to the altitude property.
If the altitude is 100 m, and the vertical accuracy is 10, the altitude value you are working with would be 100 +/- 10 meters.

Gualtier is correct. However, the CLLocation provided Vertical Accuracy is often itself wrong. I have tracks that have a constant claimed vertical accuracy of 3 metres, but I know that many points are 5 metres or more different from a known elevation. Furthermore, CLLocations from the iPhone/Watch Fitness app claim Vertical Accuracy of less than 1 metre, but have exactly the same errors (i.e. > 5 metres) as data from a 3rd-party iOS app using CoreLocation. When plotted, the elevation profiles are the same (and not as per reality), it's just that the WorkOut data have been "cleaned" and therefore have less variance.

I have found that CMAltimeter provides far more consistent and accurate data (+/- a few centimetres). CLLocation.altitude is way too inaccurate, and inconsistently so, for me to be able to create meaningful running course profiles; whereas CMAltimeter data are usually very close to reality, with low variance (making programatic identification of slope segments easy).

I have a chart of CMAltimeter vs CLLocation altitude comparisons, but I can't find a way of posting it in this answer.

Regards,

Michaela