A good question, and not one that has been formalised .
@JLaw set up the group for us slower riders, but as most of us are on the wrong side of the weight scale, so maybe a BMI based guide might be a help? I think I worked out my BMI to be 28+ the other day, so maybe 25+ is a guide for a Moose? Certainly puts the rider in the overweight category
The goats appear to be lightweights who I just don't think enjoy the finer things in life like us Mooses (things like pies, burgers and real ale

) - I hear Goats eat mostly salad and plain chicken