# Class ObjectIdentification

The `org.apache.sis.internal.jaxb.gco` package conceptually defines two complementary objects: `ObjectIdentification` and `ObjectReference`. However, only the latter is defined by a Java class, because the former is implicitly defined by the classes in the public API of SIS. This page contains the information that we would have put in `ObjectIdentification` Javadoc is such class existed.

## Overview

The `gco:ObjectIdentification` XML attribute group is implicitly included by all metadata types defined in the `org.apache.sis.metadata.iso` packages. The attributes of interest defined in this group are `id` and `uuid`.

This `gco:ObjectIdentification` group is complementary to `gco:ObjectReference`, which defines the `xlink` and `uuidref` attributes to be supported by all metadata wrappers in the private `org.apache.sis.internal.jaxb.metadata` package and sub-packages.

## Difference between `gml:id` and `gco:uuid`

The GML identifiers page said:

- `id` is a standard GML attribute available on every object-with-identity. It has type=`"xs:ID"` - i.e. it is a fragment identifier, unique within document scope only, for internal cross-references. It is not useful by itself as a persistent unique identifier.
- `uuid` is an optional attribute available on every object-with-identity, provided in the GCO schemas that implement ISO 19115 in XML. May be used as a persistent unique identifier, but only available within GCO context.

However, according the OGC/ISO schema, those identifiers seem to be defined in the GCO schema.