Subfield $w - Control subfield: [4XX and 5XX: Tracing Fields - Simple Cross References]
Subfield $w contains up to four character positions (designated as $w/0, etc.) that indicate whether special instructions apply to the display of the heading in the 4XX or 5XX field and whether a tracing is restricted to the reference structure of a particular type of authority. Subfield $w need not be used if none of the characteristics is applicable.
Because the definitions of the codes in subfield $w are dependent on the character position in which they occur, the coding of any character position mandates that each preceding character position contains a code or a fill character ( | ); succeeding character positions need not be used. For example, if $w/0 and $w/1 are not applicable but $w/2 is applicable, then $w/0 and $w/1 each contain code n (Not applicable) or a fill character.
The four character positions are:
$w/0 - Special relationship
$w/1 - Tracing use restriction
$w/2 - Earlier form of heading
$w/3 - Reference display