If I show available range as "Rated" (in the US, anyway), I believe this to show, based on a certain energy usage rate, how much energy is in the battery. Now, if I switch to percentage, what is the percentage showing? Is it showing percentage of original capacity, or percentage of available capacity? In other words, if I have a 5% degradation in the capacity of my battery, will it charge to 100%, or just 95%? Presumably the rated range would show the degradation (or else I would never know it's happening), but is it reflected in percentage as well?

It's just an indicator like a fuel gauge, F = Full (100%) E = Empty (0%). If you drive fast on the highway, hard acceleration starts or lots of hills to climb you get less miles of range for a Full 'tank' than easy starts and slower city speeds, just like any car behaves. If you want to know actual capacity, then you would need to make a calibrated measurement following a specific test procedure. If you wanted to determine degradation then you would make this calibrated measurement periodically and plot the capacity versus time or versus number of charge-discharge cycles.

Yes I think that is correct. It is a percentage of how much rated range you have at that moment out of the maximum possible range you would have if the battery was charged to 100% at that moment. It cannot be used to measure battery degradation. If you want to estimate degradation, compare a current maximum range on a full charge to what it was at some earlier point in time.