It really depended on the time period and the realm. For instance in 11th century England every 5 'Hides" of land had to supply a warrior with helmet and byrnie although every freeman could be called upon for a set time period. The Normans after Hastings supplied land to the nobles and church for a set number of Knights etc. Henry III chose the best 5-8 from every Ville to be part of the militia. Henry also made it so that depending on a mans yearly income he either payed a fine or served in a capacity suited to his income level. So ideally the best man was chosen from the people but as you get higher in income and later in the period Knights and other wealthier men had the potential to be very unqualified (and/or reluctant) to fight. Some wealthier individuals just payed the fine or hired mercenaries to fight in their place. I hope that somewhat answered at least part of your question. If you are really interested in the period and what went into the making of a medieval military force I recommend this book http://www.amazon.com/gp/product/0880290447/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1 It is a little dry but is a straight forward and simple read with the type of information you seek.