Subfield code $1 - Real World Object URI  (Appendix A:  Control Subfields)

Subfield $1 contains a URI that identifies a description of an entity, sometimes referred to as a Thing, a Real World Object or RWO, whether actual or conceptual.  When dereferenced, the URI points to a description of that entity.  A URI that identifies a name or label for an entity is contained in $0.

To return:

Use the key.

See also:

Appendix A:  Control Subfields