Sandboxing as a campaign style is not absolute. Some campaigns leave the players to develop their own plot hooks, some drop in plot seeds for the PCs to pick through, some wall of their sandboxes for a limited area of exploration and others leave their world wide open. There are other variations, kinda like the Heinz 57 Varieties of Ketchup.
Me, I'm looking at a walled off sandbox with seeds that will lead to a wide open sandbox as the players develop their own plot hooks.
So, how do you sandbox, or do you not sandbox at all?