Hats off to Spektrum, that was a fast response. Here is the quoted response:
RPM.
The throttle curve screen integrates the RPM telemetry when you have it. This is so that you can set up your head speed to specific RPM while editing the curve.
Please do this with the model firmly anchored.
The dashes mean that your motor isn’t running right now. If the motor were running, you’d see the actual RPM.