The ISO 19115 Standard lists the following types of extensions:
- adding a new metadata section;
- creating a new metadata codelist to replace the domain of an existing metadata element that has “free text” listed as its domain value;
- creating new metadata codelist elements (expanding a codelist);
- adding a new metadata element;
- adding a new metadata entity;
- imposing a more stringent obligation on an existing metadata element;
- imposing a more restrictive domain on an existing metadata element.
and these rules for extensions:
- Extended metadata elements shall not be used to change the name, definition or data type of an existing element.
- Extended metadata may be defined as entities and may include extended and existing metadata elements as components.
- An extension is permitted to impose more stringent obligation on existing metadata elements than the standard requires. (Metadata elements that are optional in the standard may be mandatory in an extension.)
- An extension is permitted to contain metadata elements with domains that are more restrictive than the standard. (Metadata elements whose domains have free text in the standard may have a closed list of appropriate values in the profile.)
- An extension is permitted to restrict the use of domain values allowed by the standard. (If the standard contains five values in the domain of an existing metadata element, the extension may specify that its domain consists of three domain values. The extension shall require that the user select a value from the three domain values.)
- An extension is permitted to expand the number of values in a codelist.
- An extension shall not permit anything not allowed by the standard.
If extensions are used in a metadata collection, they must be described using the MD_MetadataExtensionInformation Object (see Figure). This object includes a CI_OnlineResource that describes the extensions and information about each element in the extension.