What's the difference between the software timestamp feature and the hardware timestamp?

Hardware time-stamping is used to synchronize the PTP hardware clock to the master clock. With this feature, packets are time-stamped before they enter the kernel or user space. This avoids any delays caused by kernel or user space processing. PTP works best with the hardware time stamping support on NIC.

Software time-stamping is used to synchronize the system clock to the Master clock. This approach is used mainly by NTP.