The mar21_linking_records.pl is a very helpful plugin when linking records from the various 77x fields. It will allow you to search for the record you want to link and pull in the information from it into various subfields. I believe this feature is not known very well and it would help if we added it to the default frameworks. It doesn't alter the behaviour of the fields if not used, so no harm done :)
Got the name slightly wrong, it's marc21_kinking_section.pl, but maybe we should actually rename as _record would be more accurate.
*** This bug has been marked as a duplicate of bug 29733 ***