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A DOI (Document Object Identifier) is a unique ID assigned to a single document or article to make it easier to locate. It serves a similar purpose for individual documents as an ISSN does for an entire journal publication, or an ISBN for a book. Not all documents have a DOI assigned because it is a voluntary system, and authors or publishers must pay to register a DOI.

Here is an example: 

• DOI: 10.1007/s40262-019-00775-z