Translations:Wikidata Query Service/User Manual/249/en

The subjects and predicates (first and second values of the triple) must always be stored as URI. For example, if the subject is Universe (Q1), it will be stored as $url. Prefixes allow us to write that long URI in a shorter form: $code. Unlike subjects and predicates, the object (triple's third value) can be either a URI or a literal, e.g. a number or a string.