Are contributions to conference proceedings considered to be "archival" publications? While technical papers are generally considered to be "non-archival" and journal papers "archival", I couldn't find a clear statement about papers that appear in conference proceedings. Does it matter in which for the proceedings are published, e.g. only online, on CD/USB drive, or printed with/without ISBN?
Unfortunately, the reason you can't find a clear statement on this issue is because there isn't one.  The answer depends heavily on the field and the particular conference, and there isn't necessarily an absolute answer even in a specific case.  It may depend on why you care:


You may care yourself, because you want your paper to be carefully archived and made available in perpetuity.  In that case, it depends on whether a long-lasting, trustworthy organization has committed to providing access forever (and whether they have archiving contingency plans in case they go out of business).
You may need to make this distinction to assist in evaluating your CV.  In that case, the evaluators probably care primarily whether the papers are carefully refereed and in their final form, but the only way to know for sure is to ask them what their criteria are.
You may be wondering whether you can publish an updated version of your conference paper in a journal.  This depends on the customs in your field, and it requires agreement from both the conference and the journal.  The publishing agreement for the conference should specify what sorts of further publication are allowable (if it does not, then you should be sure to ask, since saying nothing suggests they do not expect any further publication at all), and the submission instructions for the journal may also address this issue.  If your submission is a revision of a conference paper, you should always specify this fact clearly and give a precise reference somewhere on the first page and in the cover letter, to make sure nobody could accuse you of hiding the previous publication.


The answers to these questions can vary.  For example, major theoretical CS conferences are archival in sense 1 (permanent archiving and availability) but not 2 or 3 (papers are not necessarily intended to be in their final form and definitive versions may be published elsewhere).