What's the meaning of "value class space is flat"?

I am now reading the book Programming in Scala. In chapter 11, it mentioned:

Note that the value class space is flat.

But no one explain what it means. Is it important? Why? And how to use and h开发者_如何学Pythonow to check value class space is really flat. It seems that ,it should told that ref class space is not flat, but no, and no other words say it again. So I want to know what the meaning of "space is flat", and why value class space is flat.

A flat class hierarchy is one with lots of sibling classes and few or no subclasses (none in this particular case).