The §w subfield contains a one-character alphabetic code describing the relationship between a reference and the 1XX field. A brief list of these codes is given below. You’ll find the detailed list on the MARC 21 web page http://www.loc.gov/marc/authority/ecadhome.html

• a - Earlier heading. The established heading in the tracing field is an earlier name for the heading in the 1XX field. The code may be used to generate a special instruction phrase such as search also under the later heading: in a cross reference display.

• b - Later heading. The heading in the tracing field is a later name for the heading in the 1XX field. A phrase such as search also under the earlier heading: may be generated for display.

• g - Broader term. The heading in the tracing field is a broader term than the heading in the 1XX field. A phrase such as search also under the narrower term: may be generated for display.

• h - Narrower term. The heading in the tracing field is a narrower term than the heading in the 1XX field. A phrase such as search also under the broader term: may be generated for display.