主题：【求助】应助 UDC number——到那里找？

Dewey/UDC number
Most items (except periodicals) are classified using the Dewey Decimal Classification System or the Universal Decimal Classification System. These numbers are hierarchal（分级‘分层的）, so that the (first) part of the number can be used to retrieve all items by a broader category.
For example, in UDC 631.147(1-773) A567

631: Agriculture in general
631.147: Biotechnic, ecological farming (for alterantive, intermediate and soft technology). Organic farming and farms.
(1-773) Country code : Developing countries, third world countries
A567: Call number: picked by using author's initial and sequential numbering of the letter.
A query for all holdings in agriculture would be simply 631, but organic farming would be 631.147
You can use The Dewey/UDC Number with any another field (except Subject and ISBN/ISSN) to restrict the results.
Dewey Decimal System
UDC Main Classes
https://www.unido.org/library/help/dewey.html