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.
Are there conferences that hold a status not described by the two categories above?
Yes. An example of such a conference is the ACM Conference on Human Factors in Computing Systems (CHI). They accept full papers, notes, work-in-progress short papers, posters, demos, etc. and have different selection criteria and processes for each submission type:
- Refereed content: is rigorously reviewed by members of the program committee and peer experts. The process includes an opportunity for authors to respond to referees’ critiques. Submitters can expect to receive formal feedback from reviewers. The program committee may ask authors for specific changes as a condition of publication. Papers and Notes are refereed content.
- Juried content: is reviewed by a committee but in a less rigorous process than refereed and does not include an author’s response or conditional acceptance. Juried content is generally not required to make the same level of lasting and significant contribution to our knowledge and understanding as refereed content. Authors who submit to juried tracks may expect to receive light feedback of up to a few paragraphs in length. The following tracks contain juried content: Late Breaking Work, Case Studies, alt.chi, Student Design and Research Competitions.
- Curated content is highly selective but does not necessarily follow a reviewing process by a committee. Curated content may be selected from submissions or invited by the track chairs. Authors who submit to curated tracks should not expect to receive formal feedback on their submission other than the selection decision. The following tracks contain curated content: Workshops/Symposia, Panels, Courses, Doctoral Consortium, EXPO, Special Interest Group (SIG) meetings, Videos.
(source: https://chi2018.acm.org/selection-processes/)
There is also a rather new hybrid journal-conference model, such as the one implemented in Ubicomp 2017:
New Publication Model
Starting with the 2017 edition, UbiComp no longer considers full paper or note submissions. Instead, it will invite for presentation papers published by the Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies (IMWUT). IMWUT has 1 volume per year with 4 issues per volume, published in March (Issue 1), June (Issue 2), September (Issue 3), and December 1 (Issue 4).
The UbiComp 2017 main technical tracks will consist of papers in IMWUT 2017 Issues 1-3. Authors of these papers will be invited, but are not obliged, to present at UbiComp. The conference will retain its workshop, poster, and demo tracks, which will have their own publication outlet.
To explain the change, in earlier years this conference was the category 1 conference-as-publication-venue, with peer review and proceedings published. Now full papers have been moved to a multiple deadline per year model in a journal, and authors of papers accepted to the journal are then invited - but not required - to attend and present at the yearly UbiComp conference. A number of other conferences in related fields are discussing possibilities of adopting similar styles of hybrids.
This is not a case of an already common practice of a journal inviting submissions from conference proceedings, such as turning a smaller conference paper into a full journal article. To attend and present as an author of a full paper, you have to go to the journal to get to the conference, rather than the other way around.
Note that one of the influencing reasons for all this is in multi-disciplinary fields, where publication cultures clash because - as a single example - someone from psychology and someone from computer science want to co-author a paper, but one field doesn't respect or acknowledge conferences and the other field has a variety of reasons for not wanting to use traditional journal models. There is also a variety of other concerns, but those go beyond the topic of discussion anyhow. Supporters of this method hope that it will better support fields who only count journal articles, while at the same time still supporting the conference culture of networking and dissemination of conference publishing communities.