[2] Libraries use a number of basic catalogue systems such as s Library of Congress, Dewey Decimal and MARC. Even among MARC there are slight differences as in LCMARC and CANMARC.
[3] The idea of the semantic header is to provide bibliographic information about resources and by including both the URN and a list of URLs it also provides a mapping from URN to URL.
[4] Such costs could change over time and require updating.