Tasks and related items

Hello,

SPS 2013 contains a new column type called "Related Items". For example it is used in tasks.
User can click "Add related item", point to a item in sharepoint and the item is added and viewable within the task.

Does anybody know how to add such a "related item" programmaticallyor has any useful links to msdn?

I have searched the whole Inet but can't find any useful information.
Even msdn seems to lack information about this column type at this time (or i can't find it).

Thanks in advance,

Mike