I was really unaware of the difference of the words Sauce & Ketchup until I got one when I ordered the other. Here are the differences for clarity:
Sauce Ketchup Savory thing of several kinds, chillies, cheese, BBQ, … Sweet thing made from ripe tomatoes. Seen as thinner liquids. Thicker semi-liquids. Usually served hot Is always cold. Comes as a part of the food. Just an optional additive.
In the US, when you order for tomato sauce, you’ll get a big tin can with tomato pulp. Nothing more! So next time you visit a pizza shop, you know what to ask for.