I think you are thinking of Serfs. But IIRC cities often had a different status, with burgers who had certain rights and the city's right to have markets and such. Thus recruiting from a city would be recruiting volunteers, and getting troops from from your own fief would be conscription. BUT, it would mean you could only recruit from your own fief or cities, unless you made an enemy of every lord who owned a village you recruited from, as well as making your men serfs who illegally ran away.
So I think the current volunteer system based on relation makes fine sense.