{@linkplain org.opengis.referencing.ReferenceSystem Reference systems}. The following is adapted from OpenGIS® Spatial Referencing by Coordinates (Topic 2) specification.

A reference system contains the metadata required to interpret spatial location information unambiguously. The description of an object's attributes can be done explicitly, by providing all defining parameters, or by identifier, a reference to a recognised source that contains a full description of the object.

The {@link org.opengis.referencing.IdentifiedObject} interface contains attributes common to several objects used in spatial referencing by coordinates. For example, a {@linkplain org.opengis.referencing.datum.Datum datum} name might be "North American Datum of 1983". This may have alternative names or aliases, for example the abbreviation "NAD83". Object {@linkplain org.opengis.referencing.IdentifiedObject#getName primary names} have a data type {@link org.opengis.metadata.Identifier} whilst {@linkplain org.opengis.referencing.IdentifiedObject#getAlias aliases} have a data type {@link org.opengis.util.GenericName}.

Another attribute is {@linkplain org.opengis.referencing.IdentifiedObject#getIdentifiers identifiers}. This is a unique code used to reference an object in a given place. For example, an external geodetic register might give the NAD83 datum a unique code of "6269". Identifiers have a data type of {@link org.opengis.metadata.Identifier}. In addition to the use of an identifier as a reference to a definition in a remote register, it may also be included in an object definition to allow remote users to refer to the object.

Most interfaced objects are immutable. This means that implementations promise not to change an object's internal state once they have handed out an interface pointer. Since most interfaced objects are specified to be immutable, there do not need to be any constraints on operation sequencing. This means that these interfaces can be used in parallel computing environments (e.g. internet servers).

## Well-Known Text format

Many entities in this specification can be printed in a well-known text format. This allows objects to be stored in databases (persistence), and transmitted between interoperating computer programs. The definition for WKT is shown using Extended Backus Naur Form (EBNF).