Usually seen in signage in all sorts of eating establishments.
In a restaurant, the management expects you to partake (and pay for) the food prepared by them exclusively. In the event that you might want to consume food that you have brought from home or elsewhere, the employees at the restaurant are prone to raise their eyebrows.
An unhappy conversation may follow, during which you'd be asked by the waiter to stop eating 'outside food'. If you wish to be stubborn about it, a brawl might ensue.
Waiter: Sir. Outside food not allowed, sir.