hope this is clear to you.
We have been using the ACARS clearance for about two years now. It works flying both ways, and of course works different whether you get it from Shanwick or from Gander, why standardize?
At Shanwick you can request clearance at no more than 90 minutes from your Ocean entry point. Our ACARS are preprogrammed to allow you to access the menu in a straightforward way. You go to the ATC page, press the Line Select switch (in this case next to Shanwick). You fill in the required fields, flight number and code, oceanic entry point and time requested altitude and Mach speed, and in the optional field you fill in your maximum capable altitude and alternate route you would find acceptable if you want. Then you press send. Within seconds you get a message from Shanwick that they have received your request and tells you to revert to voice clearance if you don’t get the clearance in the next 15 minutes. Usually a few minutes later you receive the clearance. You check the details, and if acceptable you press the ACCEPT prompt. You than get a message stating that your clearance has been confirmed. These ACARS messages form part of your journal and are saved. If it goes wrong at any point you revert back to voice.
Gander works a bit different but the basic jest is similar. As usual the trouble is in the small variations, but in general it works quite smooth as well.
There are also many aircraft that are ADS equipped which relieves them from the pain in the behind HF position reports. No such luck yet for us. Hope it explains it to you.
Regards, O.