Mustard goes on hot dogs and pastrami or corned beef sandwiches. Ketchup only goes on hamburgers and french fries, on anything else it’s an abomination.
We have a burger place out here called The Counter that grinds the burgers to order from beef that would normally be served as steak.
To cook the burger any more than medium rare or to cover it with ketchup would be a culinary sin.
I was very happy to find out that in Texas the standard hot hamburger sandwich comes with mustard on it. I don’t know why it is, but I don’t really like ketchup.