I think that by "reserve space" they mean that when your gauge shows empty there are actually five gallons remaining. So when you fill your tank it will only take 16 gallons. This feature makes it very hard to run out of gas. To prove it, drive your X until it runs out of gas. Have a spare gas can on hand of a given amount, say 2 gallons, to dump in so you can get to a gas station. At the pump you will be able to put in at least 19 gallons.