The IEEE / ANSI Device Numbers described in IEEE C37.2 (latest version 2022) have been a fundamental basis of describing functions in the electricity industry.

Of course the generic purpose of such "nomenclature/semantic" standards is to eliminate ambiguity and arbitrary definitions that are not necessarily understood in the same way by others.

However there are many in the industry who have not delved into a full understanding of the entire C37.2 Standard and so can fall into "common errors" simply because they have seen someone-else's proliferation of incorrect terms.

On such is Rate Of Change Of Frequency as I have posted about here: ROCOF - Device 7F (... not 81R)

Another is the subject of this post for the use of Device 64 .. sometimes incorrectly referred to as 64R, 87N or 87R.

Ironically, C37.2 Annex C titled “Device 64—Ground detector relay applications” has NO examples of 64 and only deals with N and G suffix!

64 is clearly a “restricted” zone of protection, only operating for a fault within that physical zone (usually defined by the CT locations).

64R and 87R have no direct relevant meaning under Clause 3.5 .. and if R is intended to be “Restricted” is a tautology of the meaning of 64 anyway.

87N or 87R is an intriguing interpretation.
Certainly 64 is a differential application so I can see the leaning towards the overall definition of 87.
However the use of N or R is an anomaly to the descriptions in Annex C.

Annex C clearly shows that N refers to the sum of the three line currents.
However in the case of the Restricted Earth Fault application on a star winding, it is the sum of the three line currents AND the “negative” value of neutral earth leg CT in order to get the Kirchhoff’s node summation of
ΣOut – Σin should equal zero.
For a delta winding, Restricted Earth Fault is indeed the sum of the three line currents. However it is to note that there is no primary “neutral” so an N suffix to any device number for the REF function on the delta side would be misleading.

The following diagram provides the use of N and G suffix vs 64 as applied to a delta-star transformer.

It can be seen that "N" is not necessarily a "neutral" .. it would be more accurate to say that it is a "Numeric" summation of the three line currents either as connection of the phase CT secondaries or as internal numerical summation of the relay individual phase CT inputs.