MMI is a clinical determination by a treating physician (or sometimes an independent medical examiner) that a patient has reached a stable plateau in their recovery. After MMI, additional medical care may help maintain function but won't significantly improve it.
MMI is critical to case timing. We don't settle a personal injury case before MMI because the full long-term cost of the injury isn't known. Settling early. When treatment is still ongoing. Leaves money on the table because future care, permanent restrictions, and long-term function aren't yet established.
In workers' compensation cases, MMI also triggers the calculation of permanent disability ratings, which directly drive the value of the permanent portion of the claim.