The phrase "additional amount" means more than the original or usual quantity or number. It refers to something extra or beyond what is already there or expected. Full definition
For open mortgages, the borrowers have the option of paying additional amounts. (syndicatemortgages.com)
Additional amounts for expenses and profit will be added to the total. (verywell.com)
If you opt for this rider then on death your family will get additional amount equal to term assurance rider sum assured more with the basic sum assured value. (mydailylifetips.com)