When linking names, subjects etc. from authority files in various BIB fields, some cataloguing conventions require giving additional information in certain cases.

One example for such a requirement, stemming from the German speaking countries, is to add the time and place of an auction when linking to the genre/form term “auction catalogs” in BIB field 655 (see attached screenshot). This additional information is, of course, specific for each BIB record and must not be overwritten or deleted, if at a later point in time either PTC processes said BIB record or a colleague invokes the F3 functionality.

Since different subfields might be used for various types of additional information, based on the field number and the cataloguing tradition and/or authority file linked to, it would be sensible to allow to define specific subfields to be exempt based on the field number, but also the authority file in use ($2).