What are Special Ad units in Doubleclick for Publishers (DFP)

The Special Ad Units are the like expensive, sensitive or strategic ad units that should not target accidentally in any of the line items. For Example, you may want to prevent a run-of-network remnant line items from appearing on the home page. Then you need to create a special ad unit that will serve your requirement.

If you designate an ad unit as special, a trafficker must explicitly target it, even if it’s hierarchically below the inventory that’s been targeted. No line item will serve to a special ad unit unless that ad unit has been explicitly targeted.

If you mark an ad unit that has line items delivering to it as special, any line items that aren’t explicitly targeting it will stop delivering to it. If you exclude an ad unit, everything within and beneath it will be excluded regardless of whether it's special.
Special ad units are marked with an asterisk (*) in DFP, and you can designate an ad unit as 'special' on the ad unit's settings page.

You can add special ad units to placements for reporting purposes only. If you add a special ad unit to one or more placements, line items targeted to that placement don't serve to the special ad unit unless it's explicitly targeted. Additionally, any child ad units of the special add unit need to be explicitly targeted as well, even if the child ad units aren't special ad units.

When you create child ad units under a special parent ad unit, the children aren't marked as special by default. If you want one or more of the children to be special, you must mark them as such. For example, let's assume the following hierarchy, where special ad units are marked with an asterisk (*):
Sports* > Football > Positions > Quarterback*
The Sports and Quarterback ad units were specifically marked as special, but Football and Positions were not.

What this means is:

1. If you configure the line item to be run-of-network, Sports and its children aren't eligible to serve, since Sports is a special ad unit.
2. If you target Sports, then Football and Positions are targeted but Quarterback is not.
3. If you target Football, then Positions is targeted but Quarterback is not.