856 Subfield $x - Nonpublic note:

Subfield $x contains a note relating to the electronic location of the source identified in the field. The note is written in a form that is not adequate or intended for public display. It may also contain processing information about the file at the location specified.

Example:

856 1#$awuarchive.wustl.edu$cdecompress with PKUNZIP.exe$d/mirrors2/win3/games$fatmoids.zip$xcannot verify because of transfer difficulty