MARC 21 Format for Bibliographic Data: 050: Library of Congress Call Number (Network Development and MARC Standards Office, Library of Congress)

First - Existence in LC collection: # - No information provided
Used for all call numbers assigned by agencies other than the Library of Congress.; 0 - Item is in LC
Other agencies should use this value when transcribing from LC cataloging copy on which the call number is neither enclosed within brackets nor preceded by a Maltese cross.; 1 - Item is not in LC
Used by other agencies when transcribing from LC copy on which the call number appears in brackets or is preceded by a Maltese cross. Brackets that customarily surround call numbers for items not in LC are not carried in the MARC record; they may be generated for display.
Second - Source of call number: 0 - Assigned by LC
Used when an institution is transcribing from LC cataloging copy.; 4 - Assigned by agency other than LC

$b - Item number (NR): Organizations that use the Cutter-Sanborn Three-Figure Author Table may conform to Library of Congress item number practice by applying Subject Cataloging Manual: Shelflisting conventions.

$0 - Authority record control number or standard number (R): See description of this subfield in Appendix A: Control Subfields.

$1 - Real World Object URI (R): See description of this subfield in Appendix A: Control Subfields.

$6 - Linkage (NR): See description of this subfield in Appendix A: Control Subfields.

$8 - Field link and sequence number (R): See description of this subfield in Appendix A: Control Subfields.

050 - Library of Congress Call Number (R)