A publisher can send a numeric subid in the psid (partner subid) field and then that value can be mapped that to the advertiser's field; i.e., the string would reflect something like this SubSource=__PSID__. Or if you simply want to send a source value to the advertiser, you can pull the encrypted or non encrypted pid (partner id) and map that value to the advertiser’s field; i.e., source=__PID_ENCRYPTED__ or source=__PID_NONENCRYPTED__. In this case, the pub doesn’t have to send a subid, the system will just capture and send it as the pid.
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