Conferences as publication venues – black and white or is there a grey or third way?
Yes, there's a third type where a single conference uses multiple submission categories, and submissions to one category are thoroughly reviewed and published as papers in the proceedings, whereas submissions to the other category only appear with short abstracts in the conference booklet or similar, and may also be restricted to specific sessions (such as poster sessions).

I know such conferences in mathematical oriented engineering fields. As just one example, I can mention the Vienna International Conference on Mathematical Modelling. If you look at their call for papers, they have a category "Full Contribution", where full papers are solicited, to be published in the formal proceedings after the review and presentation. But they also have categories "Discussion Contribution" and "Student Contribution", where only abstracts are submitted, and there's no formal publication associated with the submission.

There are other conferences in the same field which make this distinction as well within their submission categories.