TR0866: For each METRIC's [...] while the unit of measurement of this
display differs from the METRIC's pm:Unit, the SDC METRIC PROVIDER SHALL attach one
mpkp:DisplayedBy EXTENSION [...] to the METRIC descriptor or
state [...].
The requirement allowes to attach the mpkp:DisplayedBy EXTENSION to the descriptor and state at the same time. It does not define an "exclusive or" here.
Even the NOTEs to this requirement and the documentation in the Extension XML Schema do not forbid to attach such an EXTENSION to the descriptor and state (at the same time).
In addition the requirement does not limit the number of attached mpkp:DisplayedBy EXTENSION to one. In my opinion it should do so.
Proposal:
[...] the SDC METRIC PROVIDER SHALL attach EXACTLY one
mpkp:DisplayedBy EXTENSION [...] to EITHER the METRIC descriptor or
state [...].
