Although some of us may think that the 260 field is required in a bibliographic record, the truth is, this field is only a requirement for Serials format records; and this is exactly how the default Mandatory Fields table is set-up in MARC Report.
[The '0' in our table indicates the tag itself is 'mandatory']
However, in most cases, if a 260 field is missing in a bib record, and the record is not describing a serial, nor falls into one of those exceptional cases where this field is neither necessary nor desired, then a warning message would be welcomed by most users.
And in MARC Report, we've been wrestling with where to categorize such messages for many years.
Because of the high level of granularity required to identify the exceptional cases, these messages could not be triggered via the Mandatory Fields table. For example, there is no way in this table to specify that a 'given field or subfield is Mandatory unless it is an electronic resource'.
Thus, in the past, although the missing 260 would be categorized as a 'Mandatory' field check (ie. it would be color-coded black in the brief message panel), in reality, this message had to be generated by custom code, like any other Cataloging Check (which would be color-coded red in the brief message panel). This attempt to 'have our cake and eat it too' (flag the missing 260 as an error, but color-code it Mandatory even though it was a Cataloging Check) works very nicely until the user wants to use the Cataloging Check editor to disable these messages:
This customization would always fail because changes to the Cataloging Check table had no effect on messages that were color-coded as 'Mandatory Fields' messages!
So, to make a very long story come to a close, we have re-categorized these messages as 'Cataloging Check' messages; they will now be color-coded red in the brief message panel; and they will now be customizable in the Cataloging Check editor.
Notes
There are quite a few workarounds in MARC Report for situations like the above.
1. The user may simply right-click on the '260: Required field missing' message (color-coded black until version 238), and cancel it for all time using the 'Cancelled Messages' feature.
3. A more adventurous user may create their own cataloging check (using MARC Review, and the steps outlined here for importing that check into MARC Report), and tailor a more specific message to meet their own needs.
3. Now, at last, in version 238, the user may disable messages that say '260: Required field missing' in the cataloging check editor.