Be aware that there is sometimes online-only content from publishers which offer indexed content to libraries but the content itself can only be accessed on the publisher website (not in a database) in order to drive individual subscriptions. In other words, our databases might know about an article but can't grant access. Or, we might have online access to some of their articles that were published in print as well, but, because their other articles were published only online, they do not allow for them to be held in databases. Much like other born-digital content, our institution may not be able to get a license to such content and it may not be available via interlibrary loan (because no libraries can get a license to it), either.
To learn more about paywalls, see the relevant tab in the box below: