2.8 Directive Binding
Dynamic binding of directives must adhere to the following rules:
The for, sections, single, master, and barrier directives bind to the dynamically enclosing parallel, if one exists, regardless of the value of any if clause that may be present on that directive. If no parallel region is currently being executed, the directives are executed by a team composed of only the master thread.
The ordered directive binds to the dynamically enclosing for.
The atomic directive enforces exclusive access with respect to atomic directives in all threads, not just the current team.
The critical directive enforces exclusive access with respect to critical directives in all threads, not just the current team.
A directive can never bind to any directive outside the closest dynamically enclosing parallel.