Q: What are subscribed sources?

You have not mentioned the context in which you have come across the term "subscribed sources." However, since you have referred to e-journals and subject directories, I assume you are referring to the resources that your university library is subscribed to. Most libraries are subscribed to certain paywalled resources such as journals, different databases and information portals, etc. which have moderate to high subscription fees. This is done to give students access to these sources. Some of these sources might require username and password to log in. You can ask your librarian to provide you with these details as and when you need them.

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How can I distinguish an open access journal from a subscription one?