383 Subfield $c - Thematic index number:

Subfield $c contains a number assigned to a work in a recognized thematic index in the case of certain composers.

Example:

100  1#  $aVivaldi, Antonio,$d1678-1741.$tCimento dell'armonia e dell'inventione.$nN. 1-4

383  ##  $bop. 8, no. 1-4

383  ##  $cRV 269$cRV 315$cRV 293$cRV 297

383  ##  $cF. I, 22-25

[In this example there are many identifying numbers associated with the work.]