[plt-scheme] question about question mark

Hi,
I wonder why check syntax shows a question mark pointing from var to var in the 
following code:

(module x mzscheme
 (define var 1) ;  -------------------------------------
 (define-syntax x (syntax-rules ()  ((_) var))) ; ?<-----
 (provide x))

I am rather pleased with the certainty that the variable is bound as defined in 
the module.
I use check syntax a lot. It's a very nice tool.
But the question mark confuses me. What does it tell me?
Kind regards, Jos Koot