I’m not sure if this is a CSL or zotero issue. However if you add a
conference proceedings it defaults to a type=article.
I think it makes more sense to be a chapter style article, as the typical
conference proceedings is something like
Jones X: Zeolites and synthetic mechanisms. In Proceedings of the First
National Conference on Porous Sieves: 27-30 June 1996; Baltimore. Edited by
Smith Y. Stoneham: Butterworth-Heinemann; 1996:16-27.
Which looks much more like a chapter in a book, and indeed conference
proceedings often are a collection of papers.
Whereas an article is more like
Zvaifler NJ, Burger JA, Marinova-Mutafchieva L, Taylor P, Maini RN:
cells, stromal derived factor-1 and rheumatoid arthritis [abstract].
Rheum 1999, *42:*s250.