Protect local-only MARC fields that have a specific subfield coding
There are instances where its appropriate to put in a MARC field in the bibliographic record that only pertains to your local institution and you wouldn't want that field erased if the record was overlaid. If that MARC field was designated a local-note, using a $5 then that field would be "protected".

This idea will be reviewed by the Innovative product team for consideration in planning the upcoming product roadmap.
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Lloyd Chittenden commented
You can do this now with the load profiles. I suppose if you don't have access to load profiles, it would be nice to be able to do this from within a record.