Property:Has subobject
From Book Owners Online
"Has subobject" is a predefined property representing a container construct and is provided by Semantic MediaWiki. The container allows to accumulate property-value assignments similar to that of a normal wiki page but within a different entity space while being linked to the embedding subject. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.