A Sub-Test Execution functions exactly the same way 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.
Learn more
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 Sub Test Execution button in the Test Coverage section in the Requirement Issue View:
The Create Sub Test Execution button is only displayed if Xray can create a Sub-Test Execution for the project the covered issues is on, i.e., the sub-issue type needs to be configured for that project and it should have no required fields apart from the default ones.
Remember to Associate Tests and Test Sets to the Sub-Test Execution.