Imagine if you made a product, called it 'waterproof' and then it wasn't. Thankfully Gramicci wouldn't do you like that, so they made sure it's water-tight thanks to a 2-Layer membrane and polyurethane shielded zippers.
The body is a durable nylon with DWR coating, allowing water to bead off (remember when everyone was wearing jackets in the shower?) before it event gets to that impermeable layer. There's four pockets on the body allowing you to chuck a bunch of stuff in there and even give your backpack the flick.