My wish would be to go for IfcSpacialZone in this case, if you mean that Space A is a single entity that sits in Zone M and N, respectively touching both of them.
But it might be a bit difficult, knowing that IfcSpacialZone is a case being currently developed (Link). In addition, some software such as Archicad are still not capable of using this entity correctly (even though it exists as an option). I am not sure about other software tools.
According to the IFC4 documentation the IfcSpace would have the relation with the IfcSpatialZone via IfcRelReferencedInSpatialStructure, hence nicely structured in IFC.
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