There are two ways in which test scores are referenced: norm referencing and criterion referencing. The SSAT uses norm referencing.
Norm-Referenced Tests report whether test takers performed better or worse than a hypothetical average student, which is determined by comparing scores against the performance results of a statistically selected group of test takers (the norm group), who have already taken the exam.
Criterion-Referenced Tests are designed to measure a tester's results against a fixed set of predetermined criteria or learning standards and interpret a test taker's performance without reference to the performance of other test takers. (For example, your percent correct from a classroom math test is 90% because you answered 90% of the questions correctly. Your score is not referenced to the performance of anyone else in your class.)
Why is this important?
It's important to remember that your SSAT scores are being compared to those of a very specific group of students. The SSAT norm group consists of all the test takers (same grade/gender) who have taken the SSAT for the first time on one of the Standard Saturday or Sunday administrations in the United States and Canada over the past three years. SSAT score reports feature percentile ranks, which are referenced to the performance of this norm group. For example, if you are an 8th grade boy and your gender/grade percentile rank on the March 2016 verbal section is 90%, it means that your scaled scores are higher than the scaled scores of 90% of all the other 8th grade boys who took a Standard SSAT in the United States and Canada between 2012 and 2015.
An SSAT scaled score may have a different percentile rank from year to year, depending on the pool of students who take the SSAT, and the SSAT percentile ranks should not be compared to those you may receive on other types of standardized tests, because each test is taken by a different group of students.
The SSAT norm group is highly competitive; your results are compared only with other students (same grade/ gender) who take the SSAT to apply for admission to some of the most selective independent schools in the country.