From the NAM

 

Infantry Officer in Marching Order, 1812

Aquatint by J C Stadler after Charles Hamilton Smith, 1812.

'Marching Order' consisted of the uniform and equipment that soldiers wore and carried when on the march. The officer and men depicted here all wear grey greatcoats, but these were not normally worn in action. The rank and file rolled them up and strapped them to the top of their knapsacks. The latter contained all their possessions. Officers did not carry their own kit, which travelled with the regimental baggage.

From Charles Hamilton Smith's 'Costumes of the Army of the British Empire, according to the last regulations 1812', published by Colnaghi and Company, 1812-1815.