Simple: a day bag is a light sock, often just a sleeve, for quick protection from the sun and roof racks. A travel bag is the heavy-duty haul. It’s thickly padded, has reinforced seams, multiple handles, and sometimes even wheels. You use the day bag for the dawn patrol run to your local. You need the travel bag when you’re flying to Indo or Costa Rica, where your board needs maximum cushioning from a world of potential impacts.