At a "full service" national airline here in India:
1. For economy class meals: A standard form request goes out to the caterer around 3 days in advance listing the flights departing from the station which need to be catered and how many meals of each category need to be uploaded per flight. AVML, HNML etc. Since India has vastly varying dietary practices, Veg Meals are further broken up into South Indian Veg and North Indian Veg - especially for breakfast flights to and from Southern India. A special algorithm is used to track the consumption patterns on various flights which helps the planners determine how many North Indian/South Indian Veg meals to upload and how many Non-Veg Meals. Extra meals of each category are uploaded based on historical trend analysis.

2. Special Meal requests are sent a day in advance to the caterer. With this airline, Special Meal requests other than DBML made this late are rarely executed except out of DEL. I think the airline has made a practice of uploading atleast a couple of DBML meals on every flight just in case.

3. Airlines can use more than one caterer. For the airline in question, they use Taj-SATS Catering for what they designate as Tier-1 routes and ChefAir for Tier-II routes - a factor of cost.