A certificate is sold as a minimum and its multiples. What happens to the part of my money that isn't a multiple?

Many certificates are sold as a minimum plus set multiples, meaning the amount that enters the certificate must be a whole number of that multiple. If the sum you have in mind is not a multiple, what gets locked is the largest multiple inside your money, the return is computed on that amount only, and the difference stays outside the certificate. A hypothetical example with small figures: if the product is sold in multiples of 100 pounds and you have 950, then 900 is locked and earns the return, and 50 stays outside. That difference does not vanish, and the decision is yours: keep it liquid in your account, send it to another part such as a deposit, or change the amount you put in the certificate. Note that moving the difference to another product changes that product's amount, which can change its tier and return, so ask for a new calculation. Before locking, check the certificate's minimum and multiples in the key-facts statement, and ask the bank how much will actually be locked and where the difference will go.