SFO ARINC CPDLC Checkin
Pacific Chart Supplement (the blue book from the FAA that supplements your IFR charts) Page 61 (CPDLC) entering the KZAK FIR are required to contact SFO ARINC and provide Selcal, Departure, Destination, A/C registration and advise whether Satvoice equipped (but the later is never done).
If you are HF reporting and NOT CPDLC, then you can omit the Aircraft Registration. This is because when you are CPCLC and KZAK cannot get hold of you or gets no response to direct messages, the controller will send them to the Radio Operator to deliver via HF. If you haven't been in touch with ARINC for a number of hours and haven't switched or been advised of the current HF frequencies the RO can look you up by either registration or Selcal code only.
Remember that the RO on first contact on a long-transpacific flight can only hazzard a guess at what the HF frequencies will be assigned by a different colleague/position in 3-4 hours time based on propagation so what may be assigned, could change before you get into that region. Its not an exact science.
As some Registrations change at short notice, and some Selcal codes are used by multiple aircraft (albeit in different geographical regions normally), the RO can call you straight off the panel and find you to deliver the ATC message or clearance.