[arising] out of the relation/narration [of the relator]
The term is a legal phrase; the legal citation guide called the Bluebook describes ex rel. as a "procedural phrase" and requires using it to abbreviate "on the relation of," "for the use of," "on behalf of," and similar expressions. An example of use is in court case titles such as Universal Health Services, Inc. v. United States ex rel. Escobar
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