Just a place to store stuff; Neo pays no attention to it. A general usage pattern is context dependent business logic. Say, you want to have a method on an entity object that returns a value in a specific unit of measure. Where do you get the currently selected unit? (Not from a static I hope...) Right: you put the unit of measure into the extended properties and have the entity objects grab it from there. Of course, the same could be achieved by subclassing ObjectContext.