Allegiance is literally where you belong. This is incredibly important in a medieval world, and everyone always knew where he belonged to. You were never "Richard", you were "Richard of Ubent" (or from Ubent, if you were a commoner).
There are multiple layers of allegiance:
At the top of the hierarchy is the ruler. The ruler only belongs to a duchy if he is a duke. Dukes swear fealty to the ruler. Dukes can be lords of a region within their duchy, but this is not required, and dukes cannot be lord of a region in another duchy. Lords of a region swear fealty a duke. Lords can take an estate in their region, but this is not required, and lords cannot have an estate in a region other than their own. Knights occupy an estate.
These layers are also an indication of your status within the feudal society. The sooner you can say "I don't owe them allegiance, they owe me", the higher you are. Only the ruler can claim that the realm belongs to him and not he to the realm, but a lord (who owns a region instead of being owned by one) is higher than a knight (who only owes allegiance and is owed none).
In a feudal society, allegiance is tied in closely with Oaths. Your allegiance does not come free (unless you are a commoner, in which case you are inventory and belong to the region anyway).
In a true medieval society, the oaths and allegiances binding the society together could get arbitrarily complex. In BattleMaster, we simplify things a little. The deal is: Service for Money.
Every level of allegiance can contain a benefit. Most often, only the lowest level will. Most knights belong to a specific region and owe service to its lord in return for receiving a part of the region's tax income by claiming an estate in that region. The size of the estate, and thus the income earned, is controlled by the lord.
It is possible to change your allegiance. The main disadvantage is that doing so also means losing all advantages your old allegiance had given you.
The basic principles of allegiance changes are:
- Everyone changes on his level
- Everything and everyone below follows
A knight changes his region by swearing fealty to a new lord, but cannot directly change his duchy or realm. Any changes to duchy and/or realm follow from the region he joins. If he swears fealty to a lord of another realm for example, then his realm will change too.
A lord changes his duchy by swearing fealty to a new duke, and by doing so takes his region and all troops and knights with him. If that duchy happens to be in another realm, then everyone changes realms as well. A lord can switch duchies if, at least one region of the duchy is a neighbor of the lord's region, if the lord is in a region of the duchy, if the lord meets the duke of the duchy to in person, if the duke of the duchy visits the lord's region, or if the lord is attending a tournament where the duke of the duchy is also attending.
A duke changes his realm by swearing fealty to a new ruler, taking all regions, troops, and knights within his duchy with him.
There are a few exceptions to these rules, but these are the basics.