In some states, districts can make their own decision about which model they use for determining SLD, or they can use both. The discrepancy model postulates that students with SLDs will present with a profile of average to above average cognitive ability, but achievement in one or more areas that is significantly (statistically) below that predicted by their cognitive ability. This is a generally discredited model, as it lacks a good evidentiary base, but persists in practice because it has face validity and is easy to implement. It also has value for 2e kids, who otherwise have a hard time qualifying under RTI.
RTI (or RTII, or MTSS) requires frequent progress monitoring of academic skills. It has some ideological value, especially for low-performing kids, as it is focused on identifying children who are performing below grade-level, and remediating them, without worrying about the diagnostic category. If a student is below normative expectations, and does not respond to increasingly intense general education interventions, then they are determined to have a learning disability that requires specialized instruction. Unfortunately, if you are a 2e kid, you might have all your skills at or above grade level, but still be significantly underperforming, and so won't qualify easily under RTI.
