Commission rate
A commission rate is the percentage of each order a delivery marketplace keeps in return for sending it to you. It is charged on the order total rather than on your profit, so 30% of a £22 order is £6.60 taken before a single ingredient is paid for. Uber Eats publishes 30% excluding VAT when its riders deliver and 13% when you deliver or the customer collects; Just Eat and Deliveroo publish no UK rate at all.
In practice
The commission rate is what a delivery marketplace takes from the value of every order it passes to you. It is charged on the order total rather than on your profit, which is why a 30% rate on a £22 order removes more than the margin on some of the items in it.
Only Uber Eats publishes a UK list rate: 30% plus VAT when their riders deliver, and 13% plus VAT when you deliver or the customer collects. Just Eat and Deliveroo publish nothing — their rates are agreed per restaurant, by turnover and by whatever gets negotiated, so two takeaways on the same street can be paying different numbers for the same service. The only figure that matters to you is the one on your own contract.
Commission is not inherently bad value. It buys discovery: somebody who had never heard of you ordering tonight. It becomes expensive when it is charged on the regulars who already knew your name, which is the case worth measuring rather than arguing about.
Also called: delivery app commission, Just Eat commission, takeaway commission.