What is Cumulative Bonus?
Cumulative Bonus shall mean any increase in the sum assured granted by the insurer without an associated increase in premium.

How it is used?
The Limits under eligible health insurance Policy shall be progressively increased by slabs of 5% of the Sum Insured in respect of each claim-free year of insurance, subject to a maximum accumulation of 10 slabs of cumulative bonus. Sum Insured for the purpose of calculation of Cumulative Bonus shall be the expiring Sum Insured or the revised Sum Insured whichever is lower. Where a claim has arisen in respect of any insured person under the expiring policy, the earned cumulative bonus, if any, shall be reduced by 1 slab of cumulative bonus. However under no circumstances shall the Sum insured under the policy be reduced on account of reduction of cumulative bonus. Cumulative bonus will not be considered for settling claims for pre existing disease.

There is no standard and each policy T&C should be checked with Insurer.

Hope above helps!