Most songs available on Tracklib include the right to synchronize the new song (derivative work) containing a sample. However, tracks marked with ‘Sync on Request’ do not include the rights to synchronization of the derivative work, your new song, without prior approval of the original rightsholders. This means that when you purchase a license for these songs, the original rightsholders (original label/artist and/or publisher/songwriter) have to be contacted to provide their approval if you are to synchronize your new song to any form of TV/video/podcast/advertisement or similar. These approvals are usually handled by the sync licensee, that is the company/individual who wants to synchronize your song, who needs to contact the original rightsholders directly.