[racket] Passing info from reader level

Hi, I am creating my own language with racket, it's the same as racket
except that I can embed metadata anywhere in the code. I have implemented a
read and read-syntax function which detects the metadata and returns
(make-special-comment metadata) this seems to be the only way to remove it
from the code. The documentation on special-comments does not give any
details on what it's used for but I did see references to it in
parser-tools/lex.rkt.
My questions are:
Whats the difference in using the reader and expander layers in racket from
using lex and yacc?
How are special-comments used in racket?
Is there any way to access the metadata outside of the reader?

Thanks


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