This is a unique ID of the newsletter receiver and serves as a link between Mapp Intelligence and Mapp Engage.
This dimension is a URM category and not interesting for analysis.
If implemented correctly, the receiver ID is automatically added to the newsletter link and read on the landing page of Mapp Intelligence.
Based on this link, segments can be created in Mapp Intelligence and used in Mapp Engage for targeting contacts. Learn more in this article.