All VOP scheme participants must at least support the inter-PSP API specifications set by the EPC. This ensures SEPA wide reachability and interoperability.
However, VOP scheme participants are free to use on top of these EPC API specifications, other messaging languages with other VOP scheme participants if the PSPs concerned are all part of the same closed group.
In other words, inter-PSP API specifications set by the EPC do not exclude all other possibilities, but all PSPs must at least be able to support minimum one set of specifications (the EPC ones).
It should be noted that if VOP scheme participant bilaterally or multilaterally, e.g. within a “closed group” or community, decide to apply rules and/or technical specifications different than the ones specified in the EPC VOP scheme rulebook, then the VOP scheme Rulebook’s liability provisions do not apply.