During the time when I had the opportunity to serve clients, i noticed many people asked "So... what is the HCA score of this diamond?" or
"I found a diamond online with a HCA of 0.6!"
Because scopes are not easy to analyze for beginner buyers, most people only remember the range numbers of the HCA score, which is great for a start!
However, purchasing a real solid diamond requires you to look at both of them.
To someone new, diamond A and diamond B would look good to them. Reason being they only recognize light leakage which is found in diamond C.
Diamond A however, is a shallow cut which is over contrasted despite all the plentiful "reds". This is where the HCA comes in handy because they penalize shallow and deep cut diamonds.
IF we only looked at the HCA:
We would not know the actual light performance of the diamond. Many things could go wrong, such as poorly cut girdles, broken arrows, uneven areas of light leakage. Etc. Etc.
The best way in choosing a diamond is using the HCA only as a filter tool. Pick out those in the ideal range, and after, analyze them with a ASET scope and H&A scope.
IF you're good at the ASET though, you can forgot the HCA :)
Happy Diamond Hunting, and remember: What you pay for in a diamond is every facet and every carbon. Make sure that diamond is perfect, make sure every cent is worth it. And make sure that diamond is cut to the highest possible standard.