After work I go to a friend's workplace to pick her up (she works at a deli type place). When she or another person is making a deli sandwich for a customer if they ask for mustard she puts it on the sandwich, but some customers ask for "deli" mustard"; but it is confusing, because sometimes they want yellow, sometimes they want dijon; sometimes it's english. So what gives? I've asked her and she said that she's not sure, so she always asks which kind they want and tells them what she has and they tell her what they want. So my question is: What exactly is deli mustard and why are there so many options?