Q: My paper's status changed from 'under review' to 'editor assigned.' What does it mean?

The the status "Editor Assigned" typically indicates that an Associate Editor (AE) has been assigned for your paper. The AE is responsible for processing your paper from initial editorial screening to the final decision. It is the AE's responsibility to send your paper for peer review. Since the AE has just been assigned, it is clear that the paper has not yet been sent for peer review. Therefore, the "under review" status that was displayed soon after submission probably referred to an internal review or admin check that is usually done by editorial assistants. The purpose of this check is to see whether the journal guidelines have been followed with regard to style and formatting and whether the submission package includes all the required documents. Thus, I think that for this journal, the status "under review" is used for both internal and external review. So basically, your paper has cleared the admin check and has just been assigned to an AE. The AE will conduct the initial editorial screening to check the scientific value and relevance of the manuscript to the journal. You paper will be set for peer review only once it clears this screening.

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Tracking your manuscript status in journal submission systems