Originally, people were elevated to the nobility for services to the Empire, and the spectrum of nobility ranged from smalltime families whose ruled small villages and counties, to families of intermediate power who held somewhat larger territories and may have had special priviledges like hunting rights in certain Imperial woods, and very powerful lords who held sway over whole provinces, or held a monopoly in certain trades. During the wars against the Empire, numerous nobles switched sides and joined the emergent nations, often becoming very powerful within them; others remained loyal to the Empire and lost a great deal of their holdings in the relevant provinces.
Others are nobles who now owe their allegiance to other nations who have nonetheless. They don't sit in the Goodly Chamber, their families having been expelled when they abandoned their allegiance to the Empire, the upshot being that a fifth of the seats in the chamber are left empty and the Goodly Chambers seem awfully empty - the imposing architecture and wide echoing spaces not diminishing this impression. Within the nobility of Horizon, the balance of power has swung from the landowners to the merchantile sorts, simply because there are many nobles who no longer possess any land: a few still hold sway over the borderlands surrounding Horizon and gain a fair amount of prestige for doing so (they, after all, are guarding the borders and growing most of the city's food), but those who held domains further away from the city have lost it all. Those who had sufficient free capital have managed to enter the merchantile game and rebuild their fortunes; others, whose wealth was mainly bound up in their lands, are ruined, and it's not uncommon to hear of the corpses of nobles being found by the bailiffs having drunk themselves to death on their centuries-old wine collections, or Lords and Dukes being taken away to the debtor's wing of the Doghouse.