In what order are callbacks called?

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Suppose I am using the default global callback queue. I have two Subscrbers subscribed to different topics, with different input buffer sizes and different callback functions.

What guarantees are there for the order n which callbacks are called? Are they at least guaranteed to follow the order of message arrival at the ros node?

I am aware of the message_filters tools for ordering messages based on timestamps in the messages. I am asking here what guarantees the basic ROS facilities provide.