Hard references, whether they are pointer or owner, protect an object from being purged. Soft references do not.
In AutoCAD, block definitions and complex entities are hard owners of their elements. A symbol table and dictionaries are soft owners of their elements. Polyline entities are hard owners of their vertex and seqend entities. Insert entities are hard owners of their attrib and seqend entities.
When establishing a reference to another object, it is recommended that you think about whether the reference should protect an object from the PURGE command.