Simple cross reference:

Cross reference displays generated from the tracing fields are called simple cross references.  In a simple cross reference display, the content of the 4XX or 5XX field is the heading referred from, and, following a reference instruction phrase, the content of the 1XX field is the heading referred to.  In the tracing fields, the reference instruction phrase may be implicitly associated with either the field tag or a code in subfield $w/0 (Special relationship).  In unusual cases, the reference instruction phrase may be carried explicitly in subfield $i (Reference instruction phrase).

The definition of Tag Related Reference Instruction Phrase and guidelines for applying subfields $i and $w in 4XX and 5XX fields, as well as examples of simple cross reference displays, are in Tracing Fields:  Simple Cross References.