Postback URL - is used to send conversions to the source or any additional services. You can specify postbacks at the:
Traffic source creation page. By default, always sends conversions on this postback, regardless of whether other postbacks are specified;
General tab at the campaing creation page. If it specified, always sends conversions on this postback, regardless of whether other postbacks are specified;
Advanced settings at the campaign creation page. By default, this postbacks are working if you configured them instead of postback in general tab.
Please note that the tracker with a postback in the traffic source settings sends conversions for all campaigns at once. If you need to send conversions only for a specific campaign, use the S2S postback in its settings.
Trackwill postback example: