Identifier services are tools that assign unique and persistent identifiers to research outputs, such as articles, datasets, and researchers themselves. These identifiers help to differentiate and track research outputs across various platforms and systems. Some commonly used identifier services include Digital Object Identifiers (DOIs), Researcher IDs (such as ORCID), and International Standard Book Numbers (ISBNs). These services play a crucial role in ensuring the visibility, discoverability, and proper attribution of research outputs in the scholarly communication ecosystem.