This question asks about the NORTH ATLANTIC TRAFFIC (NAT) region ......
versus MNPS airspace.
NAT region is IFR .... from
the MNPS Operation Manual ...
Over the high seas, the NAT Region is primarily Class A airspace (at and
above FL55), in which Instrument Flight Rules (IFR) apply at all times.
Throughout the NAT Region, below FL410, 1000feet separation is applied.....
MNPS airspace ... FL285 - FL420
This question turns up with exactly the same wording.