Lords of the land (in the Country of Arduin only) come in fundamentally two varieties: those with lands and those without. Those without lands have titles only and possibly some pension based on service or family tithes from commercial or military outlets. The landed nobility all hold lands outside the royal or crown lands, and are either situated near civilized centers or in areas granted by the crown for various reasons.
Seige Lords are considered lesser nobility (lowest rank), but in actuality is a title bestowed upon those qualifying few by the crown. Those mentioned in the AGs are all placed within the boundaries of the nation of Arduin and are an important part of the strategic placement of military units between potential adversaries within, and without Arduin. A few, have analogs of similar names in other nations (areas) of the world.
Arduin (country of) has lands held by the crown (including both developed and undeveloped wild lands), siege lords, landed nobility, and lands held by races in treaty. Check out AG IX or the World Book of Khaas for more on that subject and the organization of the kingdom.