Mandatory sections in the standard have some elements that are always required for all types of geospatial data sets. Terms on the right side are either other compound elements or individual data elements. By making substitutions using matching terms in the production rules, one can explain higher- level concepts using data elements. If a producer chooses to provide information enclosed by parentheses, the producer shall follow the production rules for the enclosed information. Only for terms bounded by parentheses does the producer have the discretion of deciding whether or not to provide the information.
Extended elements are elements outside the standard, but needed by the data set producer.
If extended elements are created, they must follow the guidelines in Appendix D, Guidelines for creating extended elements to the Content Standard for Digital Geospatial Metadata.