Each school will identify under-served populations. Representatives from these groups are UiM at that school.
Applicants from the Mexican and Puerto Rican communities are widely recognized as UiM. In CA, Central Americans are UiM at several schools. South Americans and Spaniards are not generally considered UiM.
If one can't speak Spanish, they are still Latino, but potential to serve that community is dramatically diminished. Schools consider this as they weigh the application.