What do the primary and secondary reductions in reducing joints or tees mean and how are they defined?

For example, if your main pipe or big head is DN250, and the small head or reducer is DN200, it is a first-level reduction; the small head or reducer is DN150, which is a second-level reduction.

To put it simply, the one size smaller than the main pipe is the first-level reduction, and the two sizes smaller is the second-level reduction.

Of course, it must correspond to the size chart. DN50 is two levels smaller than DN80. Isn’t it DN65 in the middle?