They don't (through this route at least) know what you're buying, just where. So if you go to your gas station and buy food and no fuel it will display just the same as if you bought fuel. I've just checked one of my cards, and the instore cafe at the supermarket shows up exactly the same as buying groceries ("Grocery Stores, Supermarkets") but the petrol station*
on the same site is "Service Stations (with or without Ancillary Products)".
Companies sometimes try to use this field to enforce expense policies (like "no alcohol on expenses", "fuel only to be bought on the fuel card in the car, not your company credit card") but this causes problems for legitimate purchases (like buying sandwiches at a petrol station - the card is meant to buy food when travelling, but it looks like you bought fuel) without preventing illegitimate purchases (buy a cheap meal and an expensive bottle of wine in a supermarket). It's mainly useful for you to work out where a transaction you forgot about took place
* UK terminology