Used as a gerund-noun meaning the act or instance of topping something up.
Regular topping-up of the account will prevent service interruption.
Регулярное пополнение счёта предотвратит перебои в обслуживании.
Students were charged a topping-up fee to cover laboratory costs.
Со студентов взималась доплата для покрытия лабораторных расходов.
topping-upgerund / present participle Used attributively to describe something that serves to top up.
The government introduced a topping-up payment for low-income families.
Правительство ввело пополняющую выплату для малообеспеченных семей.