It sounds as though you are in the Army Reserve or Army National Guard.
If you are on Active Duty and need help with the form, go to the Patient Administration (PAD, or records room) section of your clinic or hospital and the PAD clerk will help you.
If you are in the USAR or ARNG, check with your unit first to see if they have done a LOD (Line of Duty) determination. If they haven't, also check with the PAD section of the hospital/clinic that took care of you. If no one has done it, ask your unit to initiate it.
The reason they are asking about it is the LOD documents that this is from your military service and the military or VA is responsible for any ongoing medical care you need as a result of the injury. If the injury is disabling and you file a VA claim about it, you may need the LOD for documentation.
If the injury occurred while you were on Active Duty or Active Duty for Training, and you were not disobeying any orders, it is considered service connected and in the line of duty. If you were disobeying orders (such as being AWOL, injury from DUI, injury while committing a crime, etc) then it MAY not be. If there is any doubt, or if you are in the USAR or ARNG, the unit does a line of duty determination. If you were obeying orders and on AD or ADT, it is in the line of duty and they will document it. If there is any question that you were injured while disobeying orders, they will make a decision about if it is severe enough to state that it was not in the line of duty (most of the time they still decide it was in the line of duty) and document it. This is also used to document if it was service-connected and in the line of duty if you were traveling to or from ADT or mobilization, etc.