categories > categories navigation > baseflag

on user-navigation-trails.xt I read:

<xar:comment>
The 'baseflag' variable allows you to skip certain categories in the trail :
  #$loop:catitems:item['baseflag']# = 0 : top level
  #$loop:catitems:item['baseflag']# = 1 : above the base category
  #$loop:catitems:item['baseflag']# = 2 : base category
  #$loop:catitems:item['baseflag']# = 3 : below the base category
  #$loop:catitems:item['baseflag']# = 4 : current category
</xar:comment>


but what's the right way and the right place to set this variable?