SOS is a common notation servers use as a line note when ringing in a ticket indicating that the guest would like the sauce, or dressing, or anything liquid, to be served on the side. The abbreviation is literally sauce on the side or salsa al lado in Spanish.
When a cook is reading a dupe, and sees, for example, burger medium, the cook will put anything wet on the side. In the case of a burger it would mean that any mayonnaise, aïoli, ketchup, mustard, or other sauce that is normally put onto the burger, should go into a metal or porcelain ramekin and be served on the side of the plate.
In fine dining, you'll also see sauce on the side applied to dressings and purées. It can be frustrating as you, the cook, knows that the artfully plated and flavor balanced menu item is now out of balance because of the SOS request, but it is what the guest wants, and it's best to keep in mind that there could be a medical condition, or a simple taste preference that you can easily accommodate with the sauce on the side request so it's nothing to get bent out of shape over.