The military has its own form of laws which are punishable under the Uniformed Code of Military Justice (UCMJ) which also falls under the United States CODE (USC). All military members fall under UCMJ for punishment or corrective action of any sort, which is what an Article 91 falls under.
An Article 91 has three categories on which a member of the military can be charged for: 1) striking or assaulting a Noncommissioned Officer, Warrant Officer or Commissioned Officer while the officer is in execution of his or her office, 2) willfully disobeying a lawful order from a Noncommissioned Officer, Warrant Officer, or a Commissioned Officer, 3) treats with contempt or language in deportment that is disrespectful towards a Noncommissioned Officer, Warrant Officer, or Commissioned Officer.
An Article 91 is in place to ensure respect of lawful orders, to protect against violence or disrespect of any nature, or insult. Only warrant officers or enlisted personnel can be punished under UCMJ for an Article 91. Category 1) of an Article 91, trying to cause any bodily harm to a Warrant Officer, Noncommissioned Officer, or Commissioned Officer with the intent to hurt or kill. Examples: raising a weapon, throwing a fist or kick or using ones body in any form, or any form of sexual assault.
Category 2) of Article 91, disobeying a lawful order, general order, or a regulation where the accused is lawfully guilty of disobeying an order or the dereliction of duty. Examples: not showing up to duty, ignoring an order from a Noncommissioned Officer, Warrant Officer, or Commissioned Officer. Category 3) of Article 91, contempt or deportment of verbal or body language shows disrespect to a Noncommissioned Officer, Warrant Officer, or Commissioned Officer. Example: swearing or cussing out, giving the finger, rolling eyes, or not showing proper respect when in uniform.