These words have to do with indicating the nature, function, purpose, disposition, etc. of anything that requires special identification, usually in the form of written or printed instructions, but also verbally or by position. At its most limited, label can indicate the affixing of a slip that contains a printed, written or visual legend. In this context, tag indicates one form of labelling in which such a legend is tied or strung to something by means of cord or wire: government regulations for labelling tinned goods; certain packages that must be tagged as well as labelled. Tag might also better suggest a symbolic identification rather than a detailing of information: tagging the diseased sheep with a red marker and healthy ones with a green marker. Used more generally, label has the wider range of application for any act of identifying something: a professor who was labelled by the students as a tiresome bore. As in this example, the word can be humorous or sarcastic, but often it points disapprovingly to unfair identifications: those who label as communistic anyone interested in social welfare. With fewer general applications, tag can suggest a looser identification: He was tagged by some as a miser and by others as a prudent man. The word may be merely a more vivid or colourful substitute for label : a professor who was tagged as a silly old bore.

Categorize is the most formal and neutral word here; it applies to the classification of something within a larger system or division: an explanation of how books are categorized under the Dewey Decimal System. Because of its earlier and still active use in philosophy and logic, the word can indicate an arrangement that is both imperative and final, but, just as often, categorize can point to any arbitrary arrangement adopted for convenience in handling large masses of data or detail. Pigeonhole suggests the same idea, but is often used, like one sense of label, to indicate disapproval for an attempt to fit something, in all its complexity, into simple and preconceived stereotypes: They were more concerned to pigeonhole a patient as a manic-depressive, schizophrenic or what-have-you than to treat his individual illness. The word may also apply to the filing away of something to be dealt with later or forgotten: he pigeonholed all proposals for changing the work schedules.