Whenever I add a reference to a book in the insert citation dialogue, click on the source to add the passage in the source to which I wish to refer, change the field from ”page” to “section”, and then enter a section number, Zotero ‘forgets’ the section number upon confirmation, but remembers that I switched from “page” to ”section”; I can then re-enter the mask used to specify page ranges (is there a proper name for this mask?) and add the section number, at which point Zotero accepts it.