The affiliate reference ID is a great way to pass data into the system that may need to be returned back on a confirmation pixel.  The parameter is AffiliateReferenceID. 

See the example below.
http://www.Tracking_URL.com?CID=XXX&AFID=XXX&AffiliateReferenceID=(Add_Value_Here)&SID=

There's no reporting on the Affiliate Reference ID.  This parameter is build for unique values only that need to be backed back to the affiliate on a confirmation pixel.  If you need SubID reporting on values being passed in, please use the SubID Value.
The Sub ID is an optional alphanumeric value that is appended to your Tracking URL.  This allows you to view conversions by these values.  This is very helpful when tracking conversions for search keywords, email lists, sub affiliate, etc... 
You have the ability to add up to ten (10) sub IDs.  When adding additional Sub IDs, the format is to separate the SubIDs with Commas. 

See the example below.

Example:  http://www.Tracking_URL.com?CID=XXX&AFID=XXX&SID=value1,value2,value3,value4,,,,

Note:  When passing a SubID, the only accepted characters are alphanumeric and dashes.  Failure to comply will result in getting an error message when trying to pull your SID report.  The maximum length of a SubID is 500 characters.  You cannot pass GUIDs in as a SubIDs.  The system will recognizes GUIDs and will remove them.  If you wish to pass a GUID to a Tracking URL, please use the AffiliateReferenceID parameter.  See the example below.
http://www.Tracking_URL.com?CID=XXX&AFID=XXX&AffiliateReferenceID=(Add_GUID_Here)&SID=value1,value2,value3,value4,,,,