The SCAT isn’t used for much. I think it’s considered rather coachable. I’ve had my children take it, and qualify for those programs, but I wouldn’t assume anyone would put a lot of weight on it other than for CTY. It is a single data point.
For GT identification in school, a more acceptable and usual alternative to the group testing (CoGAT and NNAT) is individual cognitive/psychological testing (e.g., the WISC) which can be obtained through the graduate psych department.
At http://cty.jhu.edu/ts/docs/2008SCATScoreInterp26.pdf, JHU states “The SCAT is not an intelligence test. Scores from the SCAT cannot, and should not, be compared to a child’s scores on either a group or individually administered I.Q. test. The SCAT is not an achievement test. Scores from the SCAT should not be compared to a child’s scores on a test of achievement or academic content knowledge” and also “A low score on the SCAT does not mean that your child is “not gifted.” Likewise, a high score on the SCAT test does not necessarily mean that your child will be identified as “gifted” by his or her school.”