The location of a resource instance is an absolute URI constructed from the server base address at which the instance is found, the resource type and the Logical ID, such as (where 123 is the Logical Id of a Patient resource). When the location is an HTTP address, this address can generally be used to retrieve or manipulate the resource. Note that implementations SHOULD NOT assume that the location of a resource is always resolvable to an accessible server - it may be temporarily unavailable, or not available by policy (e.g. firewalls) or in some cases, it might not actually exist (e.g. use of resource outside a RESTful environment). Resources reference each other by their location. These references are allowed to be absolute or relative (see Resource References for further discussion).