The approximate number of messages in the queue that are delayed and not available for reading immediately. This can happen when the queue is configured as a delay queue or when a message has been sent with a delay parameter.
The approximate number of messages that are in flight. Messages are considered to be in flight if they have been sent to a client but have not yet been deleted or have not yet reached the end of their visibility window.
The approximate number of messages available for retrieval from the queue.