The value of this property is YES for operations that run asynchronously with respect to the current thread or NO for operations that run synchronously on the current thread. The default value of this property is NO.
In macOS 10.6 and later, operation queues ignore the value in this property and always start operations on a separate thread.
This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software.