Although I found a way to request additional information from Here-Api as described here, I am interested in how the speed data is particularly defined by. For instance, if "SP" is defined as Speed (based on UNITS) capped by speed limit, I would like to read up on what "capped by speed limit" means.
I found that this answer represents a model I am interested in. Particularly, I am interested in the difference between SU, SP, and FF.
Since the additional information provided in HERE API's XML request seems to be limited, I look for answers or links to more detailed explanations of the data mentioned above.
SP - Speed (based on UNITS) capped by speed limit. Sometimes traffic speeds can show that people are driving faster than the legal speed limit, but in this case, the speed that we use for ETA is the legal speed limit SU - Speed (based on UNITS) uncapped by speed limit. Actual speed shown by traffic speeds. The average of this speed in the traffic probe data is SU, with the data not being "capped" to speed limit and instead showing the current average speed on the link FF - Free Flow speed. It is the reference speed and is time independent.