Windows x86 physical memory is 24-bit?

Windows x86 physical memory is 24-bit?

I am reading the new book, Windows Internals 5 about memory management (chapter 9 in Physical Address Extension PAE section). It is mentioned "A 32-bit system represents physical addresses internally with 24 bits" on page 770. I am totally confused, and I think x86 physical memory should be also addressed by 32-bit, and it is why the system is called 32-bit system.

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Guest [Entry]

What this is referring to is the mapping of a 32-bit virtual address space to a 24-bit physical address space. General a 20-bit virtual page number gets translated to a 12-bit physical page number and the remaining bits are the offset into the page