The Logstash Pipeline APIs are APIs added as part of the system indices project to provide application specific APIs that encapsulate system index access. The original plan was that these APIs were not going to be documented or have a REST API spec file as they are for internal use. However, upon further thought since these APIs could be useful outside of inter-stack use and there would be benefits for the stack use such as type safety in the client used within Kibana, a decision was made that the APIs should be documented and have a REST API spec.