Adding New Item Categories
Zotero does not have the category of Advertisement under the list of new items. Can this or any other category be added manually?
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In general, you only need an item type where different citations are required - which I'd doubt is the case for an ad - instead, categorize it by the medium it's in - a newspaper or magazine article, a radio or TV broadcast, an "artwork" (e.g. a photo taken of an ad on a street) etc.
For organizing items use tags and/or collections.
I would only call something a magazine article when it's actually published in a volume - regardless of whether that volume is online only or not. E.g. blogpost up at NYTs or the like are, to me, webpages/blogposts.
I'd call a craigslist ad a forum post (internally I don't think that's distinguished from a webpage, so don't worry about terminology too much).
Again, Zotero categories are mainly bibliographical, not categorical. As you get into a subject matter, there are dozens, maybe hundreds of distinctions that you can make about item types. You can make those in your own database with tags, collections etc. but they're not of bibliographical relevance.
None of the style guides (APA, CMoS, etc.) seem to have a standard for ads.
When writing about the culture of recreational risk-taking I find that ads can be enlightening.
For the Web ads I put something like the following in the title field of the Zotero webpage item:
Advertisement for: product, company, (ad-serving or -management company); With: ArticleTitle
Advertisement for: Sonicare, Philips, (DoubleClick); With: "Some hard lessons about college costs"
I also grab and keep a snapshot or screen capture. The ads that accompany any web page are likely to be different depending upon a viewer's browsing history or some other variable.
Printed sources, such as magazines, are often regionally distributed with different ads for different regions, different ads for mailed subscriptions vs. store-bought issues, etc. In my experience, the article pagination is usually stable across versions but not always.