The T-score used in ImPACT Pediatric is a standard score with a mean of 50 and a standard deviation of 10. The T-scores are derived from the normative data gathered from the standardization sample.
To help understand the relationship between T-Scores and Percentile Ranks, you can use the score converter in the resource section of ImPACT Toolkit. You plug in the T-score and it will provide you the percentile equivalent. Alternatively, you can also look at the picture of the normal curve below. For example, a standard score on the Visual Memory Composite of 40 is 1 Standard Deviation below the mean, but within normal limits.