Any Item can be linked to one or more other Items and the links will appear on both Items that have been related.  

The link can be given a description to explain what relationship exists between the Items for other users.  

The link may be used to navigate from one Item to the other and vice versa. 

To establish a link navigate to the Items list in a Project and open the edit dialogue for one of the Items you wish to relate.  In this dialogue select the tab with the two interlinked ovals. 

This will display any existing links and give you the opportunity to create a new one by clicking on the Plus icon above the heading Title. 

This allows you to locate the Item to which you wish to link the Item that is in the edit dialogue.  You can narrow down your search by identifying the Item Type to reduce the length of the drop down list of potential Items.  Select the item and give the link a description. 

Save the link and it will appear in the links tab on both Items.