Improvement of steel cleanness requires detailed knowledge about the formation of non-metallic inclusions. Tracing inclusions and potential sources is an effective tool for studying inclusion evolution. In this article, two tracing approaches are evaluated. First, classical tracer experiments are performed, where rare earth elements (REE) like lanthanum or cerium are deliberately added to the melt on an industrial scale to mark the inclusions directly. Second, the natural REE multielement fingerprint of the inclusions is compared to the REE multi-element pattern of potential sources such as alloying and auxiliary materials. These REE distribution profiles are interpreted by normalizing the REE fractions to reference values. REE contents are analyzed by inductively coupled plasma mass spectrometry in all cases. Both approaches are applied to track the source of clogged material in the submerged-entry nozzle (SEN) from Ti-alloyed ultralow-carbon (Ti-ULC) steel production. Results show a clear connection between inclusions resulting from deoxidation and the clogged layer in the SEN.