Hi Kennedi,
A floor (or ground) reaction AFO is designed to leverage the ground reaction force moment at the ankle to increase knee stability in stance. Floor reaction AFOs limit ankle dorsiflexion during stance phase to thereby reduce knee flexion by controlling the amount of tibial advancement over the foot during the second rocker of gait. These braces have a solid ankle and anterior tibial support.
Generally, Modified Ashworth Scale grade of 3 or greater are considered severe spasticity. What qualifies as significant spasticity likely varies from individual to individual.