The name most likely stems from the Early modern warfare tactics of fighting in a line, either as an infantry or naval formation.
[2] Officers who are not line officers are those whose primary duties are generally in non-combat specialties including (depending upon the service) attorneys, chaplains, civil engineers, health services professionals, and logistics and financial management specialists.
[3] Line officers wear an inverted gold star above their rank stripes on their dress blue uniforms and, in the case of Captains (US pay grade O-6/NATO OF-5) and below, on their shoulder boards in whites.
Staff corps officers wear their corps insignia, rather than the line officer star, placed over their sleeve/shoulder board stripes on their dress blue and dress white uniforms, and on their left collar point on khakis and utility/working uniforms in lieu of matching pin-on rank insignia on the right collar point.
[3] In the United States Air Force (and USAF Reserve), officers assigned to the medical, nurse, dental, medical services (healthcare administration), biosciences, Judge Advocates and chaplain corps are professional officers.
They wear the US Coast Guard shield in lieu of the inverted star of US Navy line officers on their shoulder boards and above the sleeve braid on dress uniforms.
[9] The expression "line officer" is no longer current in the Royal Navy and Commonwealth affiliates.