A Custodian is a specialized bank which takes over the clearing and settlement as a service.

A Custodian is a bank for a bank.

A Custodian is trading with the deposits and securities of it's clients. 

A Custodian is a specialized branch of the Central Bank responsible for Clearing and Settlement.

A Custodian is not allowed to risk the deposits and securities of it's clients. 

A Custodian can keep the dividends of the shares, held for it's clients as a fee.

A Custodian is a specialized bank which takes over the processing of transactions as a service.