A TBI would be classified as mild if the result is a loss of consciousness and/or a brief change in mental status, such as confusion, memory loss, disorientation lasting less than 30 minutes. A mild TBI could be a concussion, minor head injury or trauma, minor brain injury, and so forth.
As far as coding and documentation guidelines, I'd say it all boils down to finding the appropriate code to describe the type (eg, concussion) and that falls into the less than 30 minutes time frame. You should also pay attention to any associated signs and symptoms reported as well as the circumstances that caused the injury (such as a fall).