852 $x - Nonpublic note:

Subfield $x contains a note relating to the location of the item identified in the field. The note is not written in a form that is adequate for public display. A note for public display is contained in subfield $z (Public note).

A nonpublic note that applies to a specific part of the holdings is contained in subfield $z (Public note) of the 863-865 Enumeration and Chronology field relating to the part. Example: