60 shots on a target with rings 1 - 10. This gives you the 539 (on average a 9). The 10 ring has an inner smaller ring, the x ring. It's used to break ties, depending on how many of the 10's were "perfect". The x ring usually means that the bullet hit the dead center of the target at least partially. So, in your example, the guy had 18 x shots (10's dead center).
But the x shots are only the tie breaker, 600 points no x beat 599 (59x).