XML documents typically refer to external entities, for example the public and/or system ID for the Document Type Definition. These external relationships are expressed using URIs, typically as URLs. However, if they're absolute URLs, they only work when your network can reach them. Relying on remote resources makes XML processing susceptible to both planned and unplanned network downtime. Conversely, if they're relative URLs, they're only useful in the context where the were initially created.