Related Contracts can be linked/unlinked in a few different ways but have the same three fields to relate the contracts. Only one "related contract will show up on the summary tab but if there's others, you can add them from the Related Contract tab. 

1. You can relate a contract from the Summary Tab under Contract Attributes:

2. When adding a new contract manually or using a template, you can relate another contract at the bottom of the screen.

3. From the Related Contracts tab you can click the blue + to add a new contract to relate to this existing one. 

To Link related contracts:

Related to is Archived? - This is asking if you're relating the current contract to one that's been moved to the 'archived' stage. The default for this field is False.
What Contract Type? - is requesting to know the contract type of the contract you wish to relate to. You can choose the contract type (if you know it) to narrow down the options presented in the last field. If you don't remember, select 'All'
Which Contract? - asks what contract specifically you're relating to this one. Choose the contract you wish to relate to and hit Save. Now you will see the Related Contract's name in the "Related Contract" field of the summary tab.

To Unlink Related Contracts:

Click on the words "Related Contract" in the summary of a contract container, this will reset all drop-down lists to the following criteria:

Drop Down 1: Related To is Archived?
Drop Down 2: What Contract Type?
Drop Down 3: Which Contract?

Be sure to hit Save. This will disassociate the contracts.