In Colorado there is a 5 level system. All trauma can go to lower levels, they will be stabalized and transported to nearest appropriate level.
Level I facilities provide care for the most severely injured patient. They must have a trauma team (with a neurosurgeon, cardiothoracic surgeon, and subspecialty care providers) in house around the clock. Level I hospitals must demonstrate commitment to public education and outreach programs and perform trauma research. There are currently three Level I centers in the state of Colorado: Denver Health Medical Center, St. Anthony's Central and Children's Hospital which is a pediatric trauma center.
Level II is similar to a Level I designation with the exception of research. Level II provides an on-call trauma team with 24-hour physicians. The big difference here from level I is that the trauma surgeons are not in house 24/7. Our doc has 20 minutes to arrive once called.
Level III facilities must have a full-time trauma team on-call around the clock. They are not required to provide comprehensive services through subspecialty units and are not required to provide neurosurgical care. Level III trauma centers can provide prompt assessment, resuscitation, emergency surgery, and stabilization. They also arrange for transfers to a facility that can provide more specialized trauma care.
Level IV facilities have physicians but may not have surgeons on staff. These facilities resuscitate, stabilize, and transfer patients requiring a higher level of care, based upon patient triage practices within each facility and state guidelines.
Level V facilities rules are not yet complete. Currently, Level V facilities receive and are accountable for injured persons, but are not likely to be open 24-hours a day. Accountability will include transfer agreements to transport patients to Level I to Level III facilities as appropriate.
Other states may be different, I am not sure. Most states will use a 5 level system, although they may not have centers designated down to level 5.