A new Column type "Related Items" has been Introduced in SharePoint 2013. This is a very useful column from Business prospective. After you Create an Item you can simply add a reference to another Item or a document in any List\Library as a related entity to this Item using Related Items column.

Add "Related Items" Column in a list?
You can add this column in any list or library from the Site Columns using "Add from Existing Site columns" in a list.

Steps -

1. Navigate to the list and click on "List Settings" (under List tab).

 2. In List Settings page click on "Add from Existing Site columns" under Columns.

 

3. Next Select the group where you are showing the Related Items column in Site Columns and then add it to the “Columns to add” box on right and click ok.

 

4. Now you have a "Related Items" column added to the list.

 

5. Next navigate to the list and click Create a new Item. You wont see this column in NewForm.aspx page. Once you create a new Item click the call-up menu (...) and the View Item.


6. In View Page you would see "ADD RELATED ITEM"

7. Once you click on Add Related Item you would see a popup with list and libraries select the Item or document you need to as as related entity.

And here is the resulting Item.