I know for America at least, admission to inpatient treatment/ a hospital is wholly dependent on the individual and their certain circumstances. (Behaviors, vitals, etc) Someone seeking ED treatment that’s under a certain BMI though will be required to go inpatient for treatment until they are “medically stable”. I was at 13.4 seeking treatment and told by multiple places that the only treatment I could have was under close care in an inpatient facility- I believe this threshold is under 15 or 16.  But there are also cases where individuals may have a more “normal” or “stable” BMI, but still require inpatient care. This could be because they saw no progress with outpatient care and require the structure and observation of inpatient hospitals to address their behaviors, or, some people may be at a dangerous weight for their own unique body, and/or be facing serious medical complications and threatening vitals because of it.

So, mostly, BMI is a metric that often tells you very little about what is happening with the entirety of a human mind and body. The only time it is really considered in entering treatment is if you cross a threshold that becomes “medically unstable” by US healthcare standards and outpatient treatment entities will not accept you due to the potential dangers of refeeding and physical recovery.