Gratuity Payment: When Does an Employer's Liability Actually Begin?

When does an employer's liability for the payment of Gratuity get triggered? Specifically, at the end of what continuous period of service?

Section 4 in The Payment of Gratuity Act, 1972
4 Payment of gratuity. —
(1) Gratuity shall be payable to an employee on the termination of his employment after he has rendered continuous service for not less than five years,—
(a) on his superannuation, or
(b) on his retirement or resignation, or
(c) on his death or disablement due to accident or disease:
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