What is refused, and why

An app may do anything the retailer's administrator can do in Desk. The install refuses only what would:

book a wrong money, tax, stock or fiscal figure;
stop the till saving a sale, the day's close or a new customer at the counter;
send the retailer's data somewhere the consent does not show;
let someone act as someone else, code running in the retailer's saves or in a viewer's browser included.

Every refusal, with the reason for it, is on What an app may not do. Everything else iVendNext checks when the install saves it, in its own words.