Flags can have three values: ? A user is requesting that a status be set. (Think of it as 'A question is being asked'.) - The status has been set negatively. (The question has been answered "no".) + The status has been set positively. (The question has been answered "yes".)
but what does “X” mean then?
By process of elimination, I’d assume it’s a different way of displaying the fourth value in the backend of Bugzilla, “unset” (mentioned in the next sentence of the link you quoted)