As an adjunct to the thread on Oudinot, the following might be helpful.
This famous unit was originally formed for the invasion of England. There were three iterations of the unit. The first formation in 1803 the unit consisted of the grenadier and carabinier companies (and later of the voltigeur companies) of the regiments that were still being organized, in training, or on garrison duty. It was disbanded after Austerlitz.
This provisional unit was reconstituted in November 1806 being composed of the elite companies of 3d battalions of regiments that were not in the campaigns in East Prussia and Poland. The companies were organized in five regiments and further organized in three brigades. Personnel shortages were made up with conscripts. The regiments were commanded by majors; each regiment had two battalions of six companies each. The company strength was one hundred all ranks.
The division was in the siege of Danzig, fought at Ostrolenka with Savary's V Corps, and with Lannes in the excellent delaying action before the main army arrived at Friedland.
With the activation of the Young Guard, the purpose of the unit became redundant. The companies that belonged to the regiments in Davout's command, the Army of the Rhine, were sent back to them. The companies belonging to regiments in Spain or in garrison at home were formed into a provisional corps of three divisions. They retained the old title of Grenadiers Oudinot but it was not an elite unit. The unit fought well at Wagram and was finally disbanded in 1810. The units were also termed Grenadiers de la Reserve, Grenadiers Reunis, and in 1809 Divisions Oudinot.
George Nafziger's essay on the subject is very helpful: