Cataloguing was a process that was once used by Judoon law enforcement to identify different species. When employed on the residents of Royal Hope Hospital, it separated humans from non-humans. This process was used by Judoon who were trying to locate a criminal, Florence Finnegan, a plasmavore who could mimic human biology, who was hiding among humans in a hospital on Earth. The Judoon scanned each lifeform's head, marking the humans who had already been scanned with a black "X" on the back of their hand.