Element Types, Custom Fields, and Sources can work closely together, but how you structure that relationship has a lot of flexibility.
Sources are optional. They are a convenient way to group and filter Elements, but they vary in the way that they group those Elements – Globals use Global Sets, Categories use Category Groups, Entries use Sections – and if you don't need them, you should be able to just leave them out altogether.
One place, where sources aren't completely non-existent, but where you will see the Craft user interface present an Element Type without any sources is if you restrict an Asset field to a single folder or an Entry Relations field to a single Section. When you trigger the field modal to select the related Assets or Entries, you will not see any sources in the sidebar, because all the Elements fall into that singular container that you set in the settings.
Building your custom Element without sources would behave in a similar way. Your Element index page and your Element Field Type will expand to full width and your user will just see your content however you wish to present it.
To your question in the comments: What should be an Element Type?
I'm sure this is a much longer discussion, but I will volunteer a vague one sentence answer!: when the benefits of using the extendable features of Element Types (sources, search, relations, etc.) outweigh the costs of developing more specific custom solutions for your needs.