HD GNSS correction data is used to improve the overall GNSS position estimate accuracy.
The achieved position estimate accuracy depends on the quality of the GNSS measurements. Using GNSS measurements from a good quality dual frequency Android phone allows sub-meter accuracy. Using GNSS measurements from a high precision GNSS module results in accuracy down to 10 cm.
HERE GNSS API provides the following:
The format of these data types is described in Data Types / HD GNSS Correction Data.