All the MARC Fields and how to fill them in are thoroughly described, with examples and everything, at the Library of Congress website: https://www.loc.gov/marc/bibliographic/ Additional information and other examples, including suggestions for the local fields like the 690 we use for local keywords, are also available at the OCLC (Online Computer Library Center) website: https://www.oclc.org/bibformats/en/0xx.html
Here is a link to the latest update of the Flemish specification of the SISO (version 6,2023): https://drive.google.com/file/d/1njOhECMiez-Yu_SRwApgjUR3BFi89yAJ/view