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