what does PTE_FILE_MAX_BITS signify?

[GoatZilla]

I'm having trouble figuring out what this define is really supposed to refer
to.  From the comments in the arch files, it seems to indicate a file offset
for pages that aren't present.  What I don't understand is the number of
bits it's usually set to -- 28 or 29.  This seems to be too many bits to
actually fit in a typical 32-bit PTE along with the type and present bits.