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.

[1] It is most commonly used when a government brings a cause of action upon the request of a private party who has some interest in the matter.

The government acts on the basis of the narration or recounting (Latin relatione) of the alleged facts by the relator.

Governments typically accept applications and commence litigation for ex rel.

[3] The term can also be used when a relative or party in privity brings suit on another person's behalf.