Accuracy
Barometric pressure changes according to elevation but also drifts due to weather. This drift decreases the overall precision - especially over extended periods. Turn on "Barometric Drift Compensation" in the Settings to compensate for some of it.
GPS doesn’t drift. However, the typical vertical accuracy is ±15 meters (45 feet) which makes it unsuitable for tracking climbing elevation.
One can only calculate the straight-up distance from a pressure sensor. For that reason, Climber’s Assistant has a setting to specify a route’s slope and even has a rudimentary tool to measure it. The closer the entered slope is to the real one, the more accurate the app can calculate the actual distance climbed. Additionally, because routes meander the actual route length is usually a bit longer than the measured one.