If you have a particular area of town that you only take large orders to, there are a number of options on how you can proceed with setting up that "Catering Only Zone"
If the catering zone itself exists of only Zip Codes that you do not normally deliver to, then my normal recommendation here is to set up all of the Zip Codes that you normally wouldn't deliver to and enter them into a Zone called Catering Only.
Then you can apply this zone out to all of your restaurants and
set the minimum order amounts to something worth your time ($100+)
set the delivery fees to something that would make sense for that Catering Zone for that restaurant you are applying ti to ($15 - $25), and
set the ETAs to something that makes sense for that restaurant per that zone as well (90+minutes).
Then normally what you could do from there is take any of the zones that you don't normally deliver standard sized orders to from restaurants. Such as if you deliver from Restaurant A to Zone A and Zone C normally for standard sized orders.
You can now apply zone B to this restaurant and set the minimums similar to that of the Catering Only Zone, thus allowing customers to place online orders for this restaurant no matter where they live, but the delivery fees and minimum orders and ETAs all make sense based on the distance.
In this image below see a delivery service that only delivers to Downtown Saint Louis. They've recently expanded to handle Catering Orders to the rest of the city from their restaurant partners located Downtown.