"Related Items" is a new Site Column that is a Part of Task Content type by default. With this Column you can add a reference (as Lookup) to another Item in same or different list or library as related entity.

This Site Column is a part of Hidden group (“_Hidden”) by default and is not be listed in Site Columns. So to make it Visible in Site Columns of the Site, modify the Task Content type and Change the group of this Column to make it Visible.

Lets look at the Steps -

Navigate to Site settings -> Site Content Types



Under List Content Types look for “Tasks”



Click on “Tasks” and you would see the following Columns with “Related Items” at the bottom.

Click on Related Items column and you would see something like below



Click on “Edit Site Column”



This is what you see here. Notice the _Hidden Group.



Click on Existing Group and then Change the Group this column belongs to. Click OK once done.



Next Navigate to Site Settings and select “Site columns”



In Site Columns select your group with Show Groups filter.





Now you can use this Site column in any list or Library.

Reference - http://www.learningsharepoint.com/2012/09/14/sharepoint-2013-related-items-site-column/