Apparently, I should read upthread before diving in...
I just took a look at the NUMATS test selection criteria, and it would appear that the %iles that they use for test selection are grade-level %iles (such as on school-administered MAP, Terra Nova, CogAT, etc.). So unless a student has prior talent search data, they pretty much recommend that all 3-6th graders take the PSAT, but that some particularly high-scoring 6th graders have the option of the SAT. For 6th grade, the only students for which the SAT is preferentially recommended are those who have 5th grade Explore results that are sufficiently high.
I am going to speculate that they use Explore results because they have internal data on how that population typically performs on the SAT vs the PSAT.
