If you spend on your Yonder card and you have a positive balance in your Top-up account, this balance will be used to automatically repay that spend. Once the Top-up balance has been used, the spend will then be borrowed from your credit limit.
e.g. if you have a £500 credit limit and a £200 balance in your Top-up Account, your spending limit will be £700.
If you spend £600, your Top-up balance will be reduced to £0 and you will borrow £400 from your credit limit. Your remaining spending limit will be £100.