Subfield code $8 - Field link and sequence number (R ) (Appendix A: Control Subfields )
Subfield $8 contains data that identifies linked fields and may also propose a sequence for the linked fields. Subfield $8 may be repeated to link a field to more than one other group of fields. The structure and syntax for the field link and sequence number subfield is:
$8<linking number>.<sequence number>\<field link type>
The linking number is the first data element in the subfield and required if the subfield is used. It is a variable-length whole number that occurs in subfield $8 in all fields that are to be linked. Fields with the same linking number are considered linked.
The sequence number is separated from the linking number by a period ( . ) and is optional. It is a variable-length whole number that may be used to indicate the relative order for display of the linked fields (lower sequence numbers displaying before higher ones). If it is used it must occur in all $8 subfields containing the same linking number.
The field link type is separated from preceding data by a reverse slash ( \ ). It is a code indicating the reason for the link and it follows the link number, or sequence number if present. No field link types have been defined in the MARC 21 Authority Format.
Example:
008/09 a [established heading record]
008/11 a [code indicating LCSH]
150 ##$aRadiography in orthopedics
750 #2$81$wb$aOrthopedics
750 #2$81$wb$aRadiography
788 ##$i<Text to be supplied>
[The two MeSH headings are the equivalent of the LCSH heading.]
To return:
Use the 
key.
See also: