It should at least specify the part about 942$n in MARC21. But then that brings up a further question : why just 942$n in marc21?
I don't know if there's more history to it than this, but I know Joshua created this feature up for us in dev_week. We needed a way to hide a certain subset of our collection from the OPAC: interlibrary loan items. We wanted to do this while keeping collection code accurate for statistical purposes and itemtype accurate for circulation rules (accurate meaning conforming to our regular rules for material type and audience). That I think explains the choice to use 942$n. It does seem reasonable to expand the feature so that you could specify itemtype(s), ccode(s), or MARC field(s). -- Owen -- Web Developer Athens County Public Libraries http://www.myacpl.org