The commonest scheme is Free text - i.e. just about anything goes. However, wherever possible a more controlled scheme should be used.
Where values are chosen from a specific list, the list is specified by a scheme
ISO 639-2/B scheme lists standard abbreviations for languages:
eng english
re french
Schemes effectively qualify the meaning of the value entered: "Festivals" in one scheme (e.g. Library of Congress Subject Headings) might cover a quite different range of phenomena from "Festivals" in another scheme. The value "33.6" means something quite different in a degrees-of-latitude/longitude scheme than it does in a UTM or cartesian coordinate scheme in meters and the date "1000" means something quite different in a BCE scheme from its meaning in a CE scheme (3000 years ago as compared with 1000 years ago).
ISO 8601 specifies a standard format for the entry of Common Era dates (including date and time).
