Subfield $w contains the system control number of the related bibliographic record preceded by the MARC code, enclosed in parentheses, for the agency to which the control number applies.  See Organization Code Sources for a listing of sources used in MARC 21 records.

Example:

800  1#  $aNegt, Oskar$tSchriften.$vBd. 2$w(DE-101b)967682460

See also:

Appendix A:  Control Subfields