The Referal URL shows the refering URL Network or the website of the Affiliate. A missing referal URL would be from Email Newsletters, Direct Type in. The Filter flags every conversion that does not have a referal URL or where the referal url is empty.
Note: For Postback Integration Referal URL needs to passed a parameter. If not passed the referal URL will be empty. Please check the Conversion Report if data is available before enabling.