Dear Joydeep,
The definition of incentive is "an additional payment (or other remuneration) to employees for increasing the output".
The payment of incentives is to recognise the extra work done by the employees. The recognition is by way of partaking in some percentage of the extra revenue generated. The percentage could be 2%, 5% etc. There is no fixed rule for the incentive. It varies from one industry to another and one company to another.
Thanks,
Dinesh Divekar