I think we have discussed this before, but, it really doesn't matter what sort of nomeclature is used.
Basically if it's in the red, it's BAD,
If it's in the green it's GOOD.
The actual RPM can be seen very easily by referring to your pilots handbook in the machine.
Why the honorable Fank used these numbers I know not, I mean it could have been five fifths of f' all, or, big mob, E for enough or F for f'all again. there you go. go figure.