Definition
A market interface is the point where your own system meets the commons. It is neither optional nor negotiable: the formats, the protocols and the validations are set by whoever owns the interface, and a company that wants to switch supplier for a customer or report a meter reading has to do it through the interface as it stands.
That makes the interfaces one of the few hard boundaries in an otherwise flexible IT landscape. Inside its own walls a company can choose its architecture fairly freely. Facing the market there is no freedom, only conformance or rejection. This is also why an interface is the natural place to measure whether something works: either the counterparty accepts what you send, or it does not.