Do you have funds in your account that you want to keep to finalize a transaction? Do you not want them to be used to pay for other services (for example, automatic renewal of domains or auctions won in the meantime)? You can reserve this amount in your balance. It will not be used until you release the reserve or you indicate the purpose of funds.
By selecting the operation waiting to be paid from the list, you can indicate that it will be covered with funds from the reserved amount. You can create even more than one reserve, for example to earmark one amount for renewal of domains, and another one for buying domains on the marketplace.