I finally found a true definition of the term "special exempt".
I tried emailing the ATP official site but their address came back to
me as 'undeliverable'.
This was from the TMS Cincinnati site.
One spot can be left open in case a player who would have had to qualify reaches the semifinals at Tennis Masters Canada (played the week before Cincinnati).
Why? That player can't play qualifying in Cincinnati if he's still playing
in Canada, and reaching the semifinal of a Tennis Masters Series event
should be enough to get you in the next big event.
That explains why Enqvist received a special exemption into Marseille.
He reached the semi-final of Memphis Kroger St. Jude last week.