Payments life assurance companies add to a 'with-profits' endowment. Usually made annually, possibly with a final (terminal) bonus when the endowment comes to the end of its term. Bonuses aren't guaranteed and the amount awarded can change each year.
Bonuses and/or profit-sharing allowances may be included in the income where they can be verified for the past three years at similar levels with the likelihood they will continue in the future.
In the context of an endowment policy, it is the bonuses that will give you the profits at the end of the term. This is why it is important not to cash in an endowment in the early years.