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.