iOS GCD: DISPATCH_QUEUE_PRIORITY_BACKGROUND, what does "disk I/O is throttled" mean in document?

As the document says:

DISPATCH_QUEUE_PRIORITY_BACKGROUND Items dispatched to the queue will run at background priority, i.e. the queue will be scheduled for execution after all higher priority queues have been scheduled and the system will run items on this queue on a thread with background status as per setpriority(2) (i.e. disk I/O is throttled and the thread’s scheduling priority is set to lowest value).

The last part of the document, what does "disk I/O is throttled" mean here?

Does it mean that tasks running at DISPATCH_QUEUE_PRIORITY_BACKGROUND level can't access disk?