We wanna make use of IEEE 1588 (PTP), but I am not sure what is the use Ethernet PTP target time high and low registers. And how is it used to get the PTP trigger output

[KMark.1]

For PTP implementation in STM32F427 we have decided to use PTPd stack but I am not able to understand what is the use of Ethernet PTP target time high and low registers in achieving time synchronization.