Cataloguing
Cataloging is a process used by law enforcement to identify different species, namely Humans and non-Humans.
This process is used in an investigation where police are looking for an alien criminal who is hiding amoung Humans. Because the criminal is amoung Humans, it is most likely that the criminal is a shapeshifter. Cataloging is done by scaning the brains of all suspects. Though there may be more than one alien hiding amoung Humans that isn't a criminal, if police find a non-Human, it is most likely the criminal they are after. In some cases, as soon as the police find the criminal, he or she is executed right then and there.
If police find a Human, they are marked by a black "X" drawn on their right hand. In cases where a Human has had facial contact with a non-Human, the police will increase there scans. If it turns out the Human is a Human, they are marked by an "X" and receive compensation for the trouble.
The Judoon, who serve as intergalactic police-for-hire, used cataloging during their search for the Plasmavore Florence Finnegan who had killed the Child Princess of Padrivole Regency 9. During their cataloging, the also found the Doctor and almost executed him.(DW: Smith and Jones)
