Tasks can become “Blocked” when they cannot proceed before some external requirement is satisfied. This should always be denoted with a “dependency” relationship in Asana. If the blocker is external, use a sub-task to represent it.
This is also common during review. When you get a task ready for QA, create & assign a sub-task to represent the QA pass. Assign it to the intended QA tester, then copy the link and mark it as a blocking dependency for the main task.