Optional elements are provided at the discretion of the metadata producer. The optionality of a section or compound element always takes precedence over the elements that it contains. Once a section or compound element is recognized by the data set producer as applicable, then the optionality of its supordinate elements is to be interpreted. Mandatory sections in the standard have some elements that are always required for all types of geospatial data sets. Extensibility Extended elements may be defined by a data set producer or a user community.
Extended elements are elements outside the standard, but needed by the data set producer.