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Schema.org is much richer in items than Google's documentation. For example Quotation or Diet.
Google is one of the founders of schema.org, though it is independent.
Could you tell me whether schema.org's itemscopes that are not mentioned in Google documentation are at least taken into consideration by Google when ranking sites?
The presence of structured data is not a direct signal of Google's search rank. However, this data makes it possible to represent the entities of the content of an individual page in SERP's rich results, when the conditions for creating relevant data are met that do not contradict Google's policy for SD. This in turn can increase the traffic of targeted users to your website.
The absence of some type of SD in Google support does not mean that your markup for this type will be ignored. This may mean that this type is not widely used at the present time. Your SD can be used by the API Google Knowledge Graph and for voice search. In addition, SD simply represents the content entities of each page as machine-readable content.
Google doesn't use any structured data markup for ranking purposes. Google uses a few types of structured data to give rich snippets -- special display in the search results. See Explore the Search Gallery and Rich Results | Search Central for all the ways that Google uses structured data.
Google also says that they use structured data to "understand what the page is about." I think that means that they feed structured data into their machine learning algorithms that power their semantic analysis. However, that doesn't mean that sites rank better when they implement structured data. I've seen no ranking boost when sites implement structured data.
From the perspective of a site owner, Google ignores structured data that doesn't lead directly to a rich snippet for the site. There is no SEO benefit of implementing structured data that is listed on schema.org but not in Google's documentation.
Even if you implement structured data that is in Google's documentation, there is no guarantee that Google will show a rich snippet for your site. In fact for many sites, Google decides not to show rich snippets at all until the site builds up enough trust signals. You might implement the structured data and then see no benefit from it.
The other wrinkle is that not all rich snippets are created equal. Some lead to higher click-through-rate (CTR) than others. It is worth implementing structed data for user-reviews and recipes if you have those because those rich snippets look great and drive significantly more clicks. Rich snippets for articles or breadcrumbs have a very minor impact on CTR and its probably not worth the time to implement the structured data for them.
