Short answer: medieval armies were not organized in nearly such a standard fashion, and army composition in general was different regionally and throughout the middle ages.

Longer answer:

During the First Crusade, the Frankish army was composed of several distinct and individual armies led by different "princes." Bohemond of Taranto's army differed greatly from that of Raymond of Toulouse's, based largely on the type of men that each leader could either attract or pay for. Bohemond, for instance, was renowned as a warrior, and Raymond was one of the wealthiest men to take the cross. Each had a different following, but Raymond's was regarded as the largest.

When it came to battle, however, each army would generally function as part of a whole. Raymond's foot would fight with Bohemond's foot, and their horse would fight with each other, etc. At the Battle of Antioch, Bohemond was appointed as the overall leader, and organized the army's foot in two divisions, one in front and one behind the undermanned cavalry (which, at that time, numbered around 200 only).

However, that's different from the organization at the battle of Dorylaeum, in which a Turkish ambush surprised the crusader army on the march, and different army groups stumbled into engagement at different times throughout the battle.

The division of power was also important. In Europe, leaders like William the Conqueror - and Bohemond and other crusade leaders, prior to the crusade - could lead based on their charisma, reputation, and record and keep a small (comparable to the size of the crusading army at the beginning of the march) army together without too much trouble with the personalities of their subcommanders. Campaigns waged at this level were usually about seizing or constructing strong points (castles and fortresses) and controlling the surrounding countryside through "ravaging" - basically, raiding and stealing food and other valuable goods to sustain the army. Pitched, decisive battles of the kind sought by commanders in the 19th century were uncommon.

Bohemond and Raymond, et al, were constantly at risk of losing some of their followers to other commanders - such as when Bohemond's nephew Tancred reneged on an agreement to march with Raymond in favor of another lord - because the power structures were actually much more fluid than the typical "feudal" structure would have us believe. A great lord or "prince" might command a large following, but that following was made up of small lords leading small groups of men that agreed to follow the prince either through familial ties, the promise of success, money, land or other promises. These agreements were not always firmly kept.

it is also important to point out that the crusades - especially the first crusade - were rather different in composition, structure, and goals than warfare typical of 11th century Europe.

That said, much of this information - much better explained and contextualized - is in Victory in the East: A Military History of the First Crusade by John France.