Simple cross reference:

In a simple cross reference display, the content of the 453 or 553 field is the number referred from, and, following a reference instruction phrase, the content of the 153 field is the number referred to. In the tracing fields , the reference instruction phrase is associated with a code in character positions /0 (Special relationship) and/or /1 (Hierarchical relationship) of subfield $w (Control subfield). In unusual cases the reference instruction phrase may be carried explicitly in subfield $i (Reference instruction phrase). Subfield $t (Topic) provides additional information about the use of the classification number.