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Looking at this page about Bibtex I see that the article entry should be used when the work has been published in a journal or in a magazine.
Suppose that I have a small manual/guide, which probably has not been published, but which totally looks like an article (in the classic Latex meaning), then I can't use the article entry in Bibtex because the journal is a mandatory field.
What kind of entry should one use in these cases?
Since you're looking to reference a manual, you may want to look into the @manual entry type. Its only required field is title, and it has the following optional fields: author, organization, address, edition, month, year, note, and key. (Any fields that are neither required nor optional will be ignored.)
You may also want to consider the entry type @unpublished; its required fields are author, title, and note, and its optional fields are month, year, and key.
If that doesn't work for you, you may want to look into BibTeX's catch-all entry type, called @misc: It has no required fields at all, and its optional fields are author, title, howpublished, month, year, note, and key.
At any rate, the @article entry type won't work for your entry because one of its required fields is (as you've already discovered) journal. If one omits the journal field from an entry of type @article, BibTeX will generate either a warning or an error message about the missing field.
Take a look here, it sports a very detailed discussion of BibTeX.
TL;DR: Use @article and add a @note saying it's (as of now) unpublished if some relatively firm publication data is at hand, else use @misc.
Well it all depends for what purposes you're making your work for. If you're making some seminar paper for you or your uni, I doubt that you need to be that strict. But if you are doing some serious work, then I guess that matters...
I'd put it as an article but I'd also see if there is some kind of entry in bibtex like miscellaneous, where you can specify that the work you're citing hasn't been published...
