UDC – (Rom. CZU) is the Universal Decimal Classification Index. UDC is a scheme used in libraries to classify the documents according to the subjects they deal with. This classification scheme divides the totality of knowledge into ten classes which in turn are subdivided into another ten subdivisions
ISBN – International Standard Book Number that uniquely identifies a title/an edition of a book. It always consists of 13 digits grouped into 5 segments of variable lenghts, separated by hyphens
ISMN – International Standard Music Number for Notated Music. ISMN contains 13 digits grouped into 4 segments of variable lenghts, separated by hyphens, thus resulting in a unique identification code for printed music publications intended for sale, storage and distribution.
ISSN – International Standard Serial Number is an eight-digit serial number, used to uniquely identify a serial publication. It is helpful in distinguishing between serials with the same title. It is a standardized numeric code, consisting of two groups of 4 digits each, separated by a hyphen, preceded by the acronym ISSN. The eighth character is the check digit of the code. It can be a number or X.