Each test already has a Base XP (B) value based on subject, grade, and test length. Performance-based tiers determine final XP:

Reading 100for100

When taking Reading tests through the 100for100 program, students will only earn XP when taking a test at a grade level higher than the highest grade they’ve already mastered.

Taking a test at a higher grade level → XP is awarded, because the student is attempting more advanced material.