Both regiments were large, so after the 1805 campaigns, a decree of 15 April 1806 stated that on campaign both the Guard Chasseurs a Cheval and the Grenadiers a Cheval would be organized in two regiments de marche, each one commanded by the two majors of each regiment, both under the colonel regimental commander.
The regiments de marche were designated as the 1st and 2d regiments of each corps.
The dragoons mentioned in the Correspondence were the remaining dismounted line dragoons that had not yet received their horses after the 1805 campaign. They were assigned to march with the Imperial Guard.
The infantry units were the new Guard regiments being raised that would become the fusiliers-grenadiers and fusiliers-chasseurs formed from cadres from the Grenadiers and Chasseurs a pied as well as the infantry velites that hadn't been commissioned yet. The enlisted men were picked from line units and the Legions Departementales. They would distinguish themselves in 1809 at Essling under two of Napoleon's Imperial ADCs when they defeated the Austrian IV Corps. They would be designated Middle Guard officially in 1812.