This feature is not fully finished, which is why it’s not well documented.
HermitePolynomialApproximation can use derivative information to produce better interpolation results. Currently, this means using velocity information to interpolate position. Cartesian properties in CZML can be specified as “cartesianVelocity” to load both position samples and velocity samples into a SampledPositionProperty.
In the current code, derivativeTypes is always an array containing the type Cartesian (the derivative of a Cartesian position is a Cartesian velocity) but in the future the types may be different. (For example, I believe the derivative of a Quaternion orientation is a Cartesian)
We used this feature in the NORAD Tracks Santa app last year to give better visual quality. Specifically, Hermite degree 3 to interpolate position with velocity, and Lagrange degree 1 to interpolate orientation.