440 Subfield $n - Number of part/section of a work:

Subfield $n contains a number designation for a part or section of a series title. Numbering is defined as an indication of sequencing in any form (e.g., Part 1, Supplement A, Book two). In music titles, the serial, opus, or thematic index number is contained in subfield $n.

Examples:

440 #0$aJournal of polymer science.$nPart C,$pPolymer symposia ;$vno. 39

440 #4$aThe Rare book tapes.$nSeries 1 ;$v5

A part or section designation that is an inversion of a named part that includes a number or a named part that includes chapter and/or verse numbers is contained in subfield $p.

Example:

440 #0$aNational Institute on Drug Abuse statistical series.$nSeries C