Question: Why can't I edit 1xx, 6xx, or 7xx fields in my catalog record?
Answer: These fields are authority controlled and you probably have the BiblioAddsAuthorities set to 'allow'. When it is set to 'don't allow' these fields will be locked and require you to search for an existing authority record.
Question: What's the relationship between 'Koha to MARC Mapping' and 'MARC Bibliographic Frameworks'?
Answer: Mapping can be defined through 'MARC Bibliographic Frameworks' OR 'Koha to MARC Mapping'. 'Koha to MARC Mapping' is just a shortcut to speed up linkage. If you change a mapping in one of these modules, the mapping will change in the other as well. (In other words, the two modules 'overwrite' each other in order to prevent conflicts from existing in Koha).
Question: Is there a limit on the number of items I can attach to a bib record?
Answer: There is no explicit limit to the number of items you can attach to a bibliographic record, but if you attach enough of them, your MARC record will exceed the maximum file size limit for the ISO-2709 standard, which breaks indexing for that record. I've found this occurs somewhere between 600 and 1000 items on a 'normal' bibliographic record.