What ties a response doc to a main doc?

I’m exporting a large amount of data out of a database I designed many many years ago. I need a key that will align some data from some response documents with the main documents that their tied to. I unfortunately never used a field that was shared between the two documents, but I vaguely remember there being some hidden field or value in Notes docs that tie one to the other. Is there a way to add this field to my export view, or am I just making this up.

thanks…

Subject: *The $REF field on the response doc has the UNID of the parent

Subject: Thanks

That’s right, thank you.