ISO 20022 is a financial standard covering messages sent between different organizations. These messages include credit transfer and direct debits as well as payment status reports and account statements.
The messages are defined by ISO 20022 Universal financial industry message scheme (UNIFI) and are based on XML (Extensible Mark-up Language). Full definition of these messages from ISO 20022 web site: http://www.iso20022.org/. SEPA payment messages are a subset of certain ISO 20022 messages.
Wiki page business rules in pain.001.001.03 show an example on what types of business rules exist in one of the messages depcited by ISO 20022.