This is a simple datatype that allows you to see document relationships and easily jump between related documents.
For example if you have a multi-lingual site and you copy a page and check the box:
"Relate copied items to original"
Umbraco creates a relationship between these two pages, this datatype will allow you to see which document the current node is related to and either delete the relationship, change it or add a relationship if one does not exist.
So for example if you create the same page in both language versions of your site and you wish to retrospectively create the relationship, this is the datatype for you!
Update - 23 / 08 / 2011
Update - 22 / 08 / 2011
Update - 19 / 08 / 2011