What is the difference between Instant Execution and Market Execution?

Instant Execution (precise execution) means that the Broker undertakes to execute the order either at the price at which the request was sent (i.e., at the price that was displayed on the chart when you clicked ‘Buy’ or ‘Sell’), or not execute it at all and get reject, if the market price will change during the order execution.

Market Execution means that the order is executed at the current market price. The Broker ensures a practically 100% guarantee that the order will be executed, but with a probability that it will not be executed at the requested price (i.e. with slippage), i.e. by current market price (if the market price will change during the order processing).