All three of these are built from dishes you already have, so set your menu up first.
Combo offers
A combo sells several items together at one price. Burger plus fries plus a drink for less than the three separately. Build it under Menu then Combo offers, pick the items, set the combo price. The cashier taps one button, and the bill still records what went out, so your stock and your item sales stay right.
Menu sets
A set is a fixed meal served as one thing: a khaja set, a thali, a lunch special. Build it under Menu sets. Unlike a combo, a set is how the dish is served rather than a discount on a basket, so it reads as its own item on the bill.
Sub-menus
A sub-menu is a separate menu that applies in a particular context: breakfast until 11, a bar list, a rooftop menu, a festival special. Build it under Sub-menus and put only the relevant items in it so the counter is not scrolling past dinner dishes at eight in the morning.
Which one do I want?
- Several items, one cheaper price, still itemised: combo.
- One plate that happens to contain several things: menu set.
- A different list of items for a different time or place: sub-menu.
If you run branches, you can also control which branch sells which item. See running more than one branch.