PHC · PTP Hardware Clock
The clock inside a NIC that a PTP daemon disciplines to the grandmaster and that the host system clock then follows; fleet-wide offset from master is what commissioning must prove.
Current numbers
ptp4l / phc2sys / ts2phcLinux PTP daemons: ptp4l distributes IEEE-1588, phc2sys slaves the system clock to the PHC, ts2phc steers the PHC from GNSS/PPS