Roger Long wrote:

It's like the question of when they go from being a boat to a ship.
There are some commonly agreed on displacement length numbers but they
shift with time and fashion. The scandalously light of fifty years
ago would be a heavy tank today. Where will it ever end?

Yes, but I see many posts where people refer to HD vs MD with no
discussion on what these are, which tells me that there are some general
ideas about them. So what are the commonly agreed upon numbers for our
current fashion?