This is useful in cases where the requested resource is not on a server you control, or you just want to handle registering the attribution source on a different server. In this case, you can specify one or more URLs as the value of attributionsrc. When the resource request occurs the Attribution-Reporting-Eligible header will be sent to the URL(s) specified in attributionSrc in addition to the resource origin. These URLs can then respond with a Attribution-Reporting-Register-Source or Attribution-Reporting-Register-Trigger header as appropriate to complete registration.