From Wikipedia got this: The first freikorps were recruited by
Frederick II of Prussia in the eighteenth century during the
Seven Years' W. They were volunteers who fought against internal and external enemies. I remember reading though during the Napoleonic Wars Freikcoprs were basically small little armies of most likely brigade size level amount of infantry and few cavarly squadrons and a battery or 2 of cannons. They were used to get behind lines and attack lines of communication and supply and used in battle as well. However, pretty sure that freikcorps were created in almost every German state and used as a militia type unit, but it seems they were better trained and organized.
Looking online I found a book by Bruce Bassett-Powel called Freikorps of the Seven Years War and the description he gives is... Although the Prussian army was mostly made up of regulars, a large number of volunteers, soldiers of fortune and freebooters were assembled into units by, usually wealthy, soldiers of fortune who offered their services to the Prussian King. These units were known as Freikorps and proved to be of limited value militarily. However, they served as line of communications troops, scouts and, more often than not, harassing forces who put the fear of God into stragglers and the civilian population. Some of these units even changed sides and both Austria and France would employ their own Free corps or Legions. Just a little more info I found on Freikorps (Legions), small military detachments, made up of volunteers and led by partisans, supported the operations of the army by raids behind enemy lines. Frederick the Great used Freikorps during the Seven Years' War (Freiregimenter,
Freibataillone), who fought alongside hussars in the little war against the Austrians. They recruited primarily among foreigners, prisoners of war, and deserters. Unlike regular troops, Freikorps were permitted to occupy houses and villages etc. It seems the Prussian Army regular line troops hated them since they robbed them and were looters and Frederick liked to use them as front line assault troops in set piece battles to be used as cannon fodder to protect his better line troops. So they could be used as raiders on independet missions, but if used in the army they make the morale of the army drop somehow if that is even possible. They would def have lower morale than line regiments.
The French army employed similar corps of volunteers and legions, like the Chasseurs de Fischer or Volontaires Étrangers de Clermont-Prince, sometimes composed of infantry, cavalry and artillery, during the War of Austrian Succession and the Seven Years' War.
General Casimir Pulaski and Colonel Charles Armand Tuffin, marquis de la Rouërie, raised and commanded legions during the American Revolutionary war. Pulaski Legion was very strong as his cavalry squadrons were some of the better led troops who fought in the south and he has a town named after him where i live haha.
So I guess in the game they could be used as garrison troops or scouting parties so used as smaller troop types. I just found it interesting because they were small little armies that consisted of infantry,cavalry, and arty. Hope this helps and sorry for the long post