Caloric restriction is an experimental tool that utilizes “undernutrition without malnutrition.”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4115619/
A simple interpretation of CR would be: “additional leanness from a normal body weight may add health and life span delaying the process of aging.”
Regarding BMI, rule of thumb is 20-ish.
CR in the NIA study reduces body weight by 25%, so the NIA CR “man equivalent” will have a BMI of 20.8 (in the lower half of the normal weight category)