These are systems with limited spatial planning ability that serve for regional or country wide trip planning. They have already implemented a web portal that offers routing in the region they are designated for. By making use of the central node web service (and possibly displaying the extended cross-border routing results via their own GUI), the coverage of these services can be extended to the effect that transnational routing can be offered. In order to communicate with other nodes of the distributed system, every local journey planner will have to develop a web service using the common OpenAPI interface, which will have to be able to transfer data in a translated format between local protocols and the ones used by the distributed system.
In case the local systems are overlapping, the system with extended coverage could cover the routing between the exchange points without the necessity to involve the IRS.