A Sub-Test Execution has exactly the same functionalities as the Test Execution issue type. The difference between them is that the Sub-Test Execution is a sub-task and can be created within the context of a requirement.
Creating a Test Execution as a sub-task of the requirement issue provides you the ability to track executions in the Agile board.
Find more about interesting usage scenarios for Sub-Test Executions at Tips for scheduling executions.
Create Sub-Test Executions
To create a new Sub-Test Execution, click the Create new Test Execution button in the Test Coverage section in the Requirement Issue View:
Remember to Associate Tests and Test Sets with the Sub-Test Execution.