A great question. I think the answer is in three dimensions: intensity, pace, choice of tool. On an intensity scale of 1-10, Light is a 1-2, medium is 3-5, heavy is 8-10. I realize there are gaps, but there are distinctions. Intensity is essentially striking power. How much muscle or speed the Mistress uses in the blow.
The pace really makes a difference. By pace, I mean number of lashes and frequency. If it is 100 lashes in an hour, that is totally different then 100 lashes in 2 minutes. A heavy pace is about one lash every 2 seconds. There is not recovery time or ability to internalize the pain, it is just a continuous wave of pain. Along with this there is duration, for how long the continuous lashes are applied before a break. Mistress Tran likes to lash to songs. She starts when a song starts and will not stop until the song is over. It can seem like a lifetime. But it really is great fun.
The tool choice is also a key; Cat verses cane, verses single tail, verses dressage, l verses tawse. By far the tawse is the worst and the Cat-o-nine the easiest. The order I have them in is the order of intensity I experience with them. One other factor is product material. By far rubber of any kind of tool hurts much more than leather. Especially a rubber whip or tawse.
I hope this helps your thinking. But you should talk with your Mistress in the pre-session discussion to agree on these terms. While all of the Mistress at the Fortress are highly skilled, they have various levels of sadist they prefer. If you want a really Heavy session, particularly in all dimensions, then I would recommend Mistresses Tran, Zhao, and Kang. If you really want to go-for-it, do a double domme combination with any combination of these three Mistresses.
Make sure you discuss if the session is mercy or no mercy. If you agree to a no mercy - no safe word session with any of these three, you will NOT be allowed to change your mind and you will get what you agreed. That is 3/4 of the fun. No control, just on the journey.
Howard