I had someone ask me these questions recently and I realized that the answer wasn't easily apparent, so I thought I would write some details about it here.
A Block Test Case is a specific type of test case that only contains open, close & exec actions, as well as calls to any procedures (QuickCalls) explicitly marked as a Block Procedure. If you try to add any other actions to the block test case, iTest displays a validation error. Creating block test cases allows for explicit enforcement of Best Practices by forcing the test case author to only author tests that are built from an already created QuickCall.
To create a block test case, click File - New - Block Test Case and follow the wizard. Block test cases are identified with the ".ibtc" extension.
To mark a procedure as a Block Procedure click on the procedure in the test case editor, expand Procedure Properties, click on the General node and ensure "This is a Block procedure" is checked. (See attached image screenshot-2016-08-05-161721.png)