What is the meaning of “value class space is flat”?

Question

Now I am reading the book "Programming" in Scala. In chapter 11 he mentioned:

Note that the value class space is flat.

But no one explains what this means. It is important? What for? And how to use and how to check the value class space is really flat. It seems that he should say that the space of class ref is not flat, but no, and no other words say it again. So I want to know what “space is flat” and why the value of a class class is flat.

Answer 1

A hierarchy with a flat class is one with many related classes and several subclasses, or without them (in this case, not one).