This is really the one thing I can't find anything anywhere about. I have some decent knowledge of modern military organizations, but I can't seem to find anything about during medieval times.
Here's a link to a post I wrote describing the Norman army of the late 11th century and another by our own /u/MI13. If they're not what you're after, let me know and I'll try to write something that more specifically addresses your question.
https://www.reddit.com/r/AskHistorians/comments/3gvi41/how_were_medieval_armies_organized/cu269h2/