Changing definitions of high-pressure steam

What was considered the lower limit for high-pressure steam?

It depended on the period, rising from about 400 to 500 and eventually around 600 by World War II.

High pressure is a moving historical category rather than one fixed engineering figure. An early threshold of roughly 400 later rose to about 500. By the Second World War, the lower boundary was around 600 as steam technology continued to improve. A pressure described as advanced in one generation could appear ordinary by the standards of a later navy.