Thanks, Gentlemen, for your comments and advice.
Yes, if I understand correctly, the CIELAB a and b channel values are not bounded, unlike for example XYZ. I was only concerned that some color, picked in RawTherapee, would be the same color if the same numbers were picked in another program. By which I mean that Lab is device-independent, of course.
I do have a tendency to lag behind the crowd on computers and software and cameras, as a grumpy old man properly should, so I'll continue with V3 for now but thanks for the tip.