I know CogAT uses standard scores; so the composite translates into a percent or easier to understand as a ratio. In this case a 158 roughly translates to only 1 in 20,000 kids score that high.

If you want to wrap your head around it more, this is a nice intro article about highly gifted+ kids at Hoagies:
http://www.hoagiesgifted.org/highly_gifted.htm